Expert on 5th gen linked brakes/slave master cylinder needed

My rear centre piston was locking up and not retracting (piston rebuild glides in and out when dry). I suspect it is a problem in the SMC or PCV. SMC piston moves freely and not leaking. The upper port with a green valve in is operating as a one way flow. Is this correct? How would the pressure from rear caliper return to the rear MC to release the centre piston? 

Also when blocking the two rear outer pistons and using the foot brake the centre piston actuated. Is this correct operating? 

I replaced lines and bled 3x but went for a ride and the rear locked with that centre piston... Struggling to know what to do. I've taken the SMC off too clean the valve a that's all I could find but would like further thoughts before I pump a litre of fluid through the bike again. 

Also what is the purpose of the (one way?) Green valve. If I remove it would the bike function the same. Should the green valve be two way? I've drawn the hydraulics system and looked at the parts diagrams and can't get my head around it. 

Also where the top SMC piston attaches to the fork there are two corroded circles of metal stuck to the fork. Should these be washers as they do not move.. Can't remove them either and doesn't appear to be a bearing at the top. Though I don't think caused the problem as when unbolting the smc/caliper from fork the rear centre piston was still unable to retract in. 

Need to fix this otherwise I will have to scrap my beloved VFR 😞

I attached pictures. The braided hoses I believe are connected correctly all other pistons operate is only the rear centre that's getting stuck. I held the SMC at 15degrees to bleed. I may have actuated the SMC during bleeding. Not sure if this has caused a problem should I actuate the SMC at all during bleed or only use the rear foot brake?

I do have a vaccum bleeder and an going to wrap ptfe tape on the bleed nipples as when I bleed it tries to suck in air. 

If you do not have the Factory Service Manual you can download it for free in the downloads section.  It has system  diagrams and troubleshooting information.   

11 minutes ago, Cogswell

If you do not have the Factory Service Manual you can download it for free in the downloads section.  It has system  diagrams and troubleshooting information.   

I have referred to this and have carried out the process but it does not mention the specific question about the SMC and the PCV aren't serviceable. That's why I'm looking for help troubleshooting before resorting to used ebay parts. 

So the rear centre piston should ONLY activate when the SMC moves under front wheel braking. 
 

If it moves when rear pedal is pressed, there is a fault with the system. IIRC the PCV should only control the rear pedal to front brake loading. 
 

if the rear centre piston locks that will be due to the SMC not releasing pressure, which implies a fault in the brake filter/return circuit. The SMC is known for this problem, due to lack of flow & fluid replacement, contamination builds in the line & blocks the filter &/or return circuit. 
 

Hope that  helps.

You can get a rebuild kit for the SMC.....

Help me with the green screen thingy... is that in the PCV, I never had that apart on mine.... kinda thinkin there shouldn't be any one-way valves in any braking system, fluid should be free to return with release of the pedal or lever.  Might be some kind of blockage somewhere in that circuit, that's why it's important to keep fluid fresh.

2 hours ago, raYzerman

You can get a rebuild kit for the SMC.....

Help me with the green screen thingy... is that in the PCV, I never had that apart on mine.... kinda thinkin there shouldn't be any one-way valves in any braking system, fluid should be free to return with release of the pedal or lever.  Might be some kind of blockage somewhere in that circuit, that's why it's important to keep fluid fresh.

The rebuild kit is just for the piston component which isn't leaking and is working correctly. The green screen is inside the SMC. There may be another in the PCV I haven't taken it apart or seen a break down of that component. The PCV I refer to is the one under the fuel tank. There's a proportional valve on the Right leg of the bike which I don't know the specifics of either.

I'll put a video underneath to show the green valve out and what I'm trying to say. I also attach a diagram I made to try and get my head around the components as the service manual doesn't explain it enough for me.

I've just seen a diagram from ST1300 forum of the SMC. Showing the small return port in SMC can be blocked, but as I was able to use my compressor to push the green thing out, I do not believe the piston is rusted and causing that hole to be blocked. Might have to ask on that forum too as they have a similar design.

Video of the SMC green part and trying to explain..

OK, I'm learning something..... I'll look closer at mine tomorrow... preliminary theory, SMC when activated is a master cylinder that must block backflow to the reservoir, just like any of the MC's.  So if there's a ball valve in there, must be how it's done.  After MC relaxes (lift your foot off the pedal, let go of the lever, or SMC comes back to rest position, it will take on more fluid from the reservoir during bleeding.  Ball valve is sticking thus rear center piston is still pressing on the pads.  Sound about right?

9 hours ago, raYzerman

OK, I'm learning something..... I'll look closer at mine tomorrow... preliminary theory, SMC when activated is a master cylinder that must block backflow to the reservoir, just like any of the MC's.  So if there's a ball valve in there, must be how it's done.  After MC relaxes (lift your foot off the pedal, let go of the lever, or SMC comes back to rest position, it will take on more fluid from the reservoir during bleeding.  Ball valve is sticking thus rear center piston is still pressing on the pads.  Sound about right?

I've wandered on to ST owners forum as one guy has written a lot about the SMC and I'm assuming it's the same design so I hope to have some more answers from there. 

What you say I'm not sure about. When the SMC piston is pushed then it will block both ports into the "mc" area so that doesn't need a one way piece. Unless the green valve is to aid in bleeding the system as that's essentially what speed bleeders are. The ST forum says to release the rear piston all the way in the caliper you must undo the bleed nipple otherwise they do not go in. This is similar to my experience. I'm guessing under the green valve in the video that chamber is enough for any heat expansion of fluid. And I'm thinking that to force and activate the centre piston using the foot pedal will 'break' the system as the system is design for the SMC to only activate that piston. Now I think when I rode after assembled everything that the SMC was not pressing during breaking and this causes a problem. Or there was air that expanded under heating. The brake did release slightly after it cooled. Going to look at the spare SMC I have and see if it's better condition and also going to check the needle bearing on the Lower fork leg today. 

I stand corrected. the inlet port cartridge (the green piece inside SMC) does have a relief hole next to the spring one way valve. It's the size of the tiniest guitar wire... I had to force the wire through and I pierced the mesh. Still don't think this is enough to allow fluid to freely release the piston from the caliper though, or maybe it was blocked or the relief port was blocked underneath in the SMC body. I can only assume this cartridge is to assist in bleeding, to stop air bubbles being sucked back (as this hole would be too small for a bubble to pass through, and the mesh would make bubbles smaller too. Otherwise not sure of the purpose since it is NOT one way... it just heavily restricts back flow.... which only would occur if you need to push the piston into the brake caliper or to relieve pressure of the system if it were expanding.

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Just to update, still experiencing rear brake lock up when riding. After the brake cooled (after about an hour) the brake would loosen up. To me this shows that the fluid heats up and expands and the return port on a master cylinder is blocked (I did also check that I didn't over-fill the brakes which is not the case). Front brakes aren't dragging. I can return rear outer pistons so the rear MC is not blocked, nor is the front brake lever MC as the return port is free. This leaves the SMC, I've ordered a rebuild kit, waiting for that to arrive. I have a spare SMC bracket from ebay which is in good condition so will use that one. Might remove the one way valve on the green cartridge after. Not sure if it's worth installing 'correctly' going through 2 litres of fluid to bleed the system then having to pull it out due to a blockage still. The green valve has a one way large hole and a 0.25mm hole going both directions. The small hole should allow for fluid expansion during heat once the SMC is not actuated however there is also a small hole in the SMC itself like with all master cylinders. My opinion is that this valve works to assist the bleeding process, allowing essentially one way movement of fluid/air. Without this during the Front left (rear circuit) bleed procedure air could continually be sucked through the rest of the rear line. The cartridge in the ST1300 Linked brakes is clear plastic, and instead of ball bearing and spring, has a rubber stopper and spring, again not documented in manual and cannot order as a spare part. The ST linked brakes uses a larger bore SMC so it is not really compatible with VFR, although cheaper and still able to be ordered, I might resort to one as it would only apply a bit more brake to the rear centre piston, and there is a PCV in the middle to stop the rear brake being over activated during heavy braking anyway. 

One more check I did was to wedge a piece of wood to push back the SMC to ensure it wasn't being activated by an incorrect pushrod length, still rear piston not returning.

Going to get circlip pliers out and remove the pushrod on SMC to see if there is anything in the piston that's causing a blockage (maybe rust or corrosion inside). After I do that it will narrow down to the green valve being the culprit.

Also ordered a used PCV valve in case that's at fault (will loosen the banjo bolt at front to check this.

Will report back with any solutions to help future users.

I went through this same drill last year with a friends 01. We ended up replacing the SMC (complete) and the issue went away. He had the exact same issue as you're experiencing, rear brake locking up. I completely tore apart the SMC and cleaned it up, down and sideways. When I opened the SMC there was a very fine pasty film on the inside of it, I believe that whatever this was must have contaminated the green item you picture above. Long story short, we found the issue to be somewhere in the SMC. Although I like your investigative approach, you'll probably end up with the same conclusion. The SMCs are still available BTW but your call as to whether you still want to do your forensics. Anyway you slice it,please update us as to your findings.

Cheers

Mohawk, 4/3/2020 at 9:04 AM

So the rear centre piston should ONLY activate when the SMC moves under front wheel braking. 
 

If it moves when rear pedal is pressed, there is a fault with the system. IIRC the PCV should only control the rear pedal to front brake loading. 
 

if the rear centre piston locks that will be due to the SMC not releasing pressure, which implies a fault in the brake filter/return circuit. The SMC is known for this problem, due to lack of flow & fluid replacement, contamination builds in the line & blocks the filter &/or return circuit. 
 

Hope that  helps.

Mohawk, I had originally that this too. However, on at least 3 different 5 Gens, the center piston does move with the rear pedal but not at the same pressure as when the SMC triggers it. I believe that the proportional valve allows it to be engaged but not fully until the SMC comes into play. I think I have video of it when I went through this with my friends 01. I'll see if I have it and post it if I do.

Here are the videos that I made when dealing with this issue. I thought I posted these when I posted on my brake thread discussing this issue. Since this one is fresh and showing up on the recent activity, I'll post them here.

Video number 2, there are three in all

Last one. I hope these help explain the system, whether they help with the actual resolution of the issue, let me know.

I would try the rebuild kit first, it may be just a swollen rubber that perhaps blocks a port, don't know as I haven't done any SMC forensics.  Have successfully replaced kits on ST1300's (leaking or corroded).... but prices new for ST or VFR are same here, see Partzilla ~U$135.  Would it be worth getting a used one from ebay would be my next thought, chances are they are fine if you can find one.

Lookie all this for cheap......

https://www.ebay.ca/itm/98-Honda-VFR800-VFR-800-Front-Brake-Assembly-B202/182212495450?hash=item2a6cb6085a:g:8OMAAMXQGQRR-ydO&vxp=mtr

36 minutes ago, raYzerman

I would try the rebuild kit first, it may be just a swollen rubber that perhaps blocks a port, don't know as I haven't done any SMC forensics.  Have successfully replaced kits on ST1300's (leaking or corroded).... but prices new for ST or VFR are same here, see Partzilla ~U$135.  Would it be worth getting a used one from ebay would be my next thought, chances are they are fine if you can find one.

Lookie all this for cheap......

https://www.ebay.ca/itm/98-Honda-VFR800-VFR-800-Front-Brake-Assembly-B202/182212495450?hash=item2a6cb6085a:g:8OMAAMXQGQRR-ydO&vxp=mtr

I'm in the UK so the parts are not in stock, $135 is quite reasonable though, better than PCV which I think is quoted £500!!! Ebay is very hit and miss here.

12 hours ago, Duc2V4

Mohawk, I had originally that this too. However, on at least 3 different 5 Gens, the center piston does move with the rear pedal but not at the same pressure as when the SMC triggers it. I believe that the proportional valve allows it to be engaged but not fully until the SMC comes into play. I think I have video of it when I went through this with my friends 01. I'll see if I have it and post it if I do.

The fact that on 5th (and 6th?) Gen you bleed the rear brake caliper using the rear MC and Rear foot pedal shows that the piston should activate by the rear pedal, but only when stationary. The force of fluid from the pedal should collapse the SMC seal and force fluid past it. The fluid should then return through the tiny hole which maybe mine is blocked or what. When the bike moves the rear pedal shouldn't over ride the SMC for centre piston, because that much force would likely trigger the PCV.

I've also just joined a CB1100xx forum that has some cool diagrams and charts for the pressures in the hydraulic system. Definitely more posts on VFR forums and ST forums than the CB and Varadero, from my minimal internet searching at least.

Good videos. I wish my bike were in such a clean condition... although I'm sure I'd still have these troubles, 59,000 miles on the clocks. I note there's aftermarket hoses, I'm going to reposition mine as I also thought the way mine are could be activating the SMC inadvertently. Something for people to be aware of when fitting aftermarket hoses (can't remember if I mentioned that). Useful that you mentioned undoing the front bleed valve and checkign the rear piston returns, I will do that, then crank the hose that goes to PCV to check that.

Also from looking at your picture it looks like you have more pushrod exposed than on mine, although as you can see in the picture I wedged the SMC far back and that's as much as I can see. 

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Did you do a rebuild kit on the SMC? When I worked on my friends bike, we did rebuild kits for all the master cylinders and all new piston seals as well. Looking at your SMC, it does appear to be in the "activated" mode, so perhaps it is stuck in that position and not allowing the fluid to return. Can you easily move the piston in and out or is it difficult? All I can tell you is that for my friends issue, it was definitely in the SMC. Hope you get it resolved.

You should be able to move the SMC on the L front caliper easily by hand when the bike is at rest. Not a lot, but you should clearly see it actuating, and it should have an obvious effect at the rear. I do this when bleeding the system periodically as normal maintenance. If not there is something wrong in there. Could be the fluid passages or even the bushings. They should be checked, and lubricated if necessary whenever the caliper is removed.

Second to last update. I rebuilt the SMC although it didn't need it IMO. The fault is the green cartridge inside. Basically I took this out, removed the one way ball bearing and so it has free fluid movement. I am very sure this is just to assist bleeding, when putting the rebuild kit in and priming it before connecting, the amount of air bubbles being generated in the unit would probably end up pushing back down the line to the front brakes. This almost one way valve would stop this problem. This would only be a problem when doing an initial bleed on the bike, so probably helps honda mechanics do the job quicker. I left the gauze in place so it would trap any dirt from entering the SMC.

Before removing the SMC again I extended the rear centre piston, undid the bleed valve on the Front Left bleed nipple, and it would not push fluid through. I then undid the bolt on the SMC out, which released fluid, ruling out the PCV unit at fault.

After I removed the one way piece (I still had to insert the green cartridge as a seal), I could easily move the rear centre piston in and out. I bled the bike and will test ride tomorrow to see if the rear locks up again under braking/heat. Now that the SMC definitely allows the fluid to expand back to the Rear MC it should be fine. Only problem is that these awful 'upgraded' lines are slightly short (picture)[image: spacer.png] and I feel it might be slightly activating the SMC. If so I will just use my old rubber honda one if there is any trouble tomorrow.

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Will report back in 24 hours with the result of the test ride.

If all is well then I at least will not shy away from owning other Honda linked bikes (blackbird, varadero, ST1300). These linked brakes seem to cause problems across all models. I would definitely recommend people focus on this area for rust prevention as well as taking the brake fluid changes extremely seriously as over  a long time it will cause problems.

So after a test ride, the rear brake was getting stuck still. Rather than deal with brake line companies over a few mm short hose I drilled the hanger that attaches above and this gives enough slack for the SMC to be fully released.

2nd test ride, front brakes weak, either pads contaminated or need to bleed. Have pads and will bleed this weekend.

Rear brake strong, stops, doesn't lock up.

Did the linked brake test, riding and coming to halt with no brakes, can touch both discs with bare hands.

Using only front brake: Front discs warm, rear disc v.hot.

Using Rear brake: Front disc warm, rear disc v.hot again, but not locking up and is free.

Spinning the rear wheel by hand gives 3/4 turn, front wheel the same. Seems normal.

So what I have learnt:

1. DO NOT upgrade your hoses using Frentubo, or (Galfer too from what I've read). Unless your hoses are leaking or obviously bulging it's not worth any of the hassle.
2. Preventative maintenance on SMC, reapply (correct) grease and check under the rubber cover for rust, even slight build up over time can cause issues.
3. Look after your pistons, during this I found corrosion on them which I replaced them all, now I'll apply ACF grease around the outer area of the pistons (not in contact with brake fluid).
4. Grease the caliper slide pins.
5. Bleeding a dry system or after unbolting a line requires a lot of fluid. I bought 4L of DOT4 and used a lot of it during this, I bled it 3 times over 2 days to make sure any air moves through the line.
6. If you still have problems with the SMC after a rebuild with return flow, remove the ball bearing from the green cartridge and re-insert (retain the filter mesh and base, as it needs to seal against the o-ring when you reassemble.

I have some cool diagrams from blackbird and ST1300 forums for reference:

If you get onto the ST owners forums, PM a guy called Igofar (Larry). He's the guru for the 1300 and linked brakes. I didn't read your post in detail but have you had the pushrod out of the SMC or changed the length? There is some magic there where the SMC has to move over a specific range to work properly and if you're out of that range it may not release pressure properly. 

Here's some recent info from Igofar (he recommends just replacing the SMC):

https://www.st-owners.com/forums/threads/st1300-secondary-master-cylinder-rebuild-smc.133460/page-3#post-2208421

Look at the size of the two ports in the SMC body (red circle around the tiny "compensating port")...

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Look at exactly where the piston needs to be for it to function properly (red lines indicate the location of the inlet port and compensating port)...

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And, consider the illustrations from the ST Owners...

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And how it fails due to corrosion and swelling of the seals...

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I'm dealing with the same issue of the rear brake locking up.

I recently rebuilt all my calipers. All sorts of brown snot was in the system. Hydraulic fluid age was "neglected" to say the least.

After the rebuild, everything seemed to be in good order (especially after a touch up bleed with speed bleeders). I had ample power in the pedal to skid the rear. Unfortunately after a few weeks, the pedal turned to total mush. Depressing the pedal to about 85% of its travel had no ill affect (but also virtually no braking effect); rear wheel can still spin relatively free. However, depressing the pedal all the way would lock the rear so the wheel couldn't be turned by hand. I could push the bike forward, and it would eventually release enough to roll the bike.

When I depress the pedal fully when riding to a complete stop, take off causes the clutch/driveline to shudder abnormally until the brake pad resistance releases (probably) by frictional wear. The rear disc is running hotter to the touch than the front.

That microscopic hole in the SMC that's so prone to failure with a bit of neglect, age, corrosion build up, tolerance stacking, etc... what a pain in the ass. Something you'd expect from German engineers. Definitely not Russian engineering.

On a positive note, the front lever braking feel and power is ABSOLUTELY dreamy after the rebuild of the calipers, MC and thorough bleed. Lever doesn't come close to the grips even on the closest setting. I love it!

Seems like the SMC compensating port blockage has been an issue for Honda for a while.

https://ridermagazine.com/2011/12/02/honda-gold-wing-gl1800-brake-recall/

https://hondanews.com/en-US/releases/gl1800-brake-recall?l=en-US&mode=print

https://blog.motorcycle.com/2015/10/21/manufacturers/honda/honda-finally-has-a-fix-for-gold-wing-rear-brake-drag-issue/

Quote

Honda now believes the problem is caused by a chemical reaction releasing a zinc compound into the brake fluid and preventing the hydraulic pressure from dropping and releasing the rear brake. Honda has developed a new secondary master cylinder that will prevent this from happening.

Could that be the fine pasty film observed by @Duc2V4?

Were there any recalls for the 5th Gen VFR's for this issue? Seems like it could apply to all LBS models.

Excellent write up Court and this saved my day, thank you.

I just bought a 98 VFR which had been sitting for 10 years so you could imagine what the brakes were like.  The SMC was totally seized as were some of the caliper pistons but was able to pull the piston out with a drilled hole and a self tapper screwed in.  Ended up rebuilding the calipers, clearing the SMC port and removing the ball and spring from the green filter and now its all good.  I just have to finish sorting the wiring as rats had made a nest in the v and chewed wires and hoses they apparently thought were in the way.

I did have a similar problem with my Blackbird some years ago but after caliper rebuilds and MUCH cleaning it was sorted.  Of course I didn't understand the green valve and SMC at that point but must have cleaned it enough to sort it.  In fact I just went out and bled the Blackbird brakes after the VFR exercise to hopefully prevent it happening again.