Rear brake locking up... rear center piston... FIXED!

So recently, I had the oh-so-common Honda (Goldwing, CBR1100XX, ST1300 and VFR800 combined braking system) rear brake lock up, dragging issue.

The rear brake pedal felt mushy like there was air in the lines, and the rear brake barely stopped the bike. However, the rear caliper was binding the rear disc. Starting from a complete stop would make the bike judder like there's something wrong with the clutch. Bike would generally feel sluggish around town. I would also smell burning brakes after coming to a stop from a sustained high speed run.

When I put the bike on the center stand and depressed the rear brake pedal, it would mush all the way in its travel. However, when I tried to spin the rear wheel, there was A LOT of resistance. After several minutes the resistance faded a bit, but there was no free spinning the rear wheel for hours. Depressed the pedal again, and rear wheel would bind again.

This illustration is EXCELLENT and helped me troubleshoot the issue. (Not sure of the original source of the diagram but I came cross it from our own Courtuk's post).

[image: chrome_PMa68JN2UP.thumb.png.c2a7601cb3757606f7468fbef02114fe.png.9558def1b7a0d56cf9298a1992f867d1.png]

After locking up the rear wheel, when I opened the bleeder for the rear outside pistons, nothing happened; wheel was still locked up. Thus, the problem wasn't with the outside pistons sticking nor the rear master cylinder which is directly plumbed (red line) to the rear outside pistons.

When I opened the bleeder for the rear center piston on a locked wheel, fluid came out (pressure released), and the rear wheel was free. Thus, the problem wasn't the center piston itself.

When I opened the bleeder for the PCV on a locked wheel, fluid came out (pressure released), and the rear wheel was free. Thus, it wasn't the line between the PCV to the center piston.

When I depressed the SMC manually on a free rear wheel, the rear wheel was locked only when the SMC was in the depressed position. Once I released the SMC, the rear wheel was free. Thus, the problem wasn't between the SMC - PCV - rear center piston (blue line).

When the front wheel was off the ground, the front wheel spun free. Thus, the problem wasn't between the RMC - delay valve - front center pistons.

Everything pointed to the inside of entry port of the SMC. Or what's known as the "green filter". Reading ST-owners forum and an anecdote from our own Duc2V4, simply cleaning the "green filter" or rebuilding the SMC doesn't solve the problem. Honda technician on ST-owners forum, Igofar, recommends replacing the SMC rather than trying to clean it or rebuild it. So, that's what I did...

06454-MBG-425 $132.94 on Partzilla when I ordered it on 4/27/2020...

[image: image0.thumb.jpeg.deb4f9b85a83471d0e4a56f8ea5dfb6d.jpeg]

[image: image2.thumb.jpeg.8612f6fafb317d749cf7510a68fb86ef.jpeg]

Here's the inlet port (green filter of the new SMC vs the one on my bike)...

[image: image5.thumb.jpeg.d6e19570ad2e5bbce95e9614b77c5488.jpeg]

Look at all that crud in there...

[image: image6.thumb.jpeg.a47d7d39f62745eb551e721a30739694.jpeg]

Here's the consequence of riding around with a sticking rear center piston... GAWD!!!

[image: image7.thumb.jpeg.c35031795347e4be9f57868c61d8d307.jpeg]

[image: image9.thumb.jpeg.699bc347ac6b78e3177ef1cc5f8c591d.jpeg]

I put everything back together, and the brakes work fantastic now. Rear disc is still warm to the touch... warmer than the front discs. I wonder if running them hot cooked the seals in the calipers. Might have to rebuild those, AGAIN!!! ARGH!!! There are reports of Goldwings bursting into flames and other bikes blowing out their brake lines from overheating the discs and consequently the calipers and the hydraulics.

Moral of the story:  If you are rebuilding brakes on a VFR800 with neglected hydraulics, Secondary Master Cylinder REPLACEMENT should be a top priority! The SMC is the weak link when the brake system is neglected. I didn't know that. I rebuilt all calipers, seals and front master cylinder, but I took my chances with an old SMC because I was ignorant. If you are refurbishing a neglected VFR, do yourself a favor and buy a new SMC before it becomes NLA.

M8. That is a GREAT write up and excellent pics, along with the excellent info you've provided. Also hi-lights the importance of regular Brake Fluid flush especially with a link brake system. What a horrible build up of gunk in the SMC!

Glad you have it all sorted. Can you get that rear disc re-surfaced?

Great update, many thanks. Looking at your old SMC is that crud or rust?

Hi, 8200rpm,

Thats a great write-up you've produced, very informative, it must have taken ages to do! (I would still be at it!)

For curiosity, I checked on the availability and price at David Silver Spares here in the UK.

The price was £233.89, (£280.69 including tax) They were also quoting $276.50 (+ tax and postage?)  So your price seems good!

Ive been into the garage and checked my own bike, rear wheel spins ok with chain drag. So maybe I'm OK for the moment.

Good to know what caused your problem. Now sorted!!

The problem with the SMC & all the additional plumbing for the linked brakes, is most users NEVER follow the maintenance schedule & never clean the components. The hydraulics on most bikes are the most neglected parts, closely followed by the suspension. 
 

They need some TLC. Brake fluid replacement schedule is every two years, but most just ignore this. Then complain when it craps out at 8 years having never been done ☹️
 

Delink does away with the problem & the extra 1.5Kg of weight the system brings. 

9 hours ago, Marooncobra

Looking at your old SMC is that crud or rust?

That reddish brown stuff is old brake fluid that turns to paste. DOT 4 should be yellow straw color like crappy light beer. Any shade of brown is not acceptable especially when you have a clog prone SMC.

This was the condition of my calipers when I rebuilt them back in December 2019...

[image: F31E3C08-F244-4C83-82C7-44791BBEA5F8.thumb.jpeg.a9d49b7f02abc1c1f2b131b267f9e823.jpg.084f0503ce0b03c74aee0d8e43583896.jpg]

Every banjo bolt I took apart had this crud in it. That "green filter" and the tiny little hole in the SMC gets clogged with this stuff. It's very difficult to clean it all out if it's stuck inside those orifices. Also, if the SMC piston seals swell out of tolerance, it could impede flow out of the "green filter".

Cleaning the green filter and rebuilding the SMC has been reported to work for some and not for others. I didn't want to take a shortcut and learn the hard way. The SMC rebuild kit is $45. I figured I'd spend a bit more and replace the entire SMC for $130.

Look at the size of this orifice... 

[image: Sawn-in-half.jpg.358cca8b2ae0eda2d9d66a8c26d3734c.jpg.6b06d47ae2be5d605bf0508cb13dc2df.jpg]

Look at the location of the holes in relation to the piston cup seal...

[image: 441732355_SMCCutoutAssembled25pc.jpg.225f3eeb9fe81ef2e30fb1240bfae7ec.jpg.005b31db3fc1e48c187b116c781d67b0.jpg]

Photos are from st-owners forum...

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

Great write up and photos... grooves will make it harder for the pads to bed...

I've had success erasing unwanted grooves in either stainless steal or cast iron
rotors by removing  the buttons... then chuck it in the Lathe... work a 180
grit sanding disc against the surface evenly until the groves are gone...

RC45 Disc Sanding
[image: 423834488_35781b3ed8.jpg]

8200

Having now seen the cut away of the SMC I think when I do my brakes next I will strip the SMC down and throw it in my ultra sonic cleaner and hit it for a good ten minutes with a brake cleaner solvent.  It may fit in a peanut butter jar so I don’t waste too much solvent during the process. That hole is tiny.

That’s a standard size bleed hole. I had a normal rear brake bleed hole block once due to some alloy corrosion. The pressure lock kept the rear brake on & eventually it stopped the bike, I was only a mile from work & thought I was losing engine power, so kept riding.  By that time the pressure was crazy from the heat & the disk was red hot. I got a spanner from some work men & opened the calliper bleed nipple it emptied the steaming fluid like a mini volcano. Just missed my head & the side of the bike thankfully. 

8200rpm, 5/10/2020 at 3:40 AM

So recently, I had the oh-so-common Honda (Goldwing, CBR1100XX, ST1300 and VFR800 combined braking system) rear brake lock up, dragging issue.

The rear brake pedal felt mushy like there was air in the lines, and the rear brake barely stopped the bike. However, the rear caliper was binding the rear disc. Starting from a complete stop would make the bike judder like there's something wrong with the clutch. Bike would generally feel sluggish around town. I would also smell burning brakes after coming to a stop from a sustained high speed run.

When I put the bike on the center stand and depressed the rear brake pedal, it would mush all the way in its travel. However, when I tried to spin the rear wheel, there was A LOT of resistance. After several minutes the resistance faded a bit, but there was no free spinning the rear wheel for hours. Depressed the pedal again, and rear wheel would bind again.

This illustration is EXCELLENT and helped me troubleshoot the issue. (Not sure of the original source of the diagram but I came cross it from our own Courtuk's post).

[image: chrome_PMa68JN2UP.thumb.png.c2a7601cb3757606f7468fbef02114fe.png.9558def1b7a0d56cf9298a1992f867d1.png]

After locking up the rear wheel, when I opened the bleeder for the rear outside pistons, nothing happened; wheel was still locked up. Thus, the problem wasn't with the outside pistons sticking nor the rear master cylinder which is directly plumbed (red line) to the rear outside pistons.

When I opened the bleeder for the rear center piston on a locked wheel, fluid came out (pressure released), and the rear wheel was free. Thus, the problem wasn't the center piston itself.

When I opened the bleeder for the PCV on a locked wheel, fluid came out (pressure released), and the rear wheel was free. Thus, it wasn't the line between the PCV to the center piston.

When I depressed the SMC manually on a free rear wheel, the rear wheel was locked only when the SMC was in the depressed position. Once I released the SMC, the rear wheel was free. Thus, the problem wasn't between the SMC - PCV - rear center piston (blue line).

When the front wheel was off the ground, the front wheel spun free. Thus, the problem wasn't between the RMC - delay valve - front center pistons.

Everything pointed to the inside of entry port of the SMC. Or what's known as the "green filter". Reading ST-owners forum and an anecdote from our own Duc2V4, simply cleaning the "green filter" or rebuilding the SMC doesn't solve the problem. Honda technician on ST-owners forum, Igofar, recommends replacing the SMC rather than trying to clean it or rebuild it. So, that's what I did...

06454-MBG-425 $132.94 on Partzilla when I ordered it on 4/27/2020...

[image: image0.thumb.jpeg.deb4f9b85a83471d0e4a56f8ea5dfb6d.jpeg]

[image: image2.thumb.jpeg.8612f6fafb317d749cf7510a68fb86ef.jpeg]

Here's the inlet port (green filter of the new SMC vs the one on my bike)...

[image: image5.thumb.jpeg.d6e19570ad2e5bbce95e9614b77c5488.jpeg]

Look at all that crud in there...

[image: image6.thumb.jpeg.a47d7d39f62745eb551e721a30739694.jpeg]

Here's the consequence of riding around with a sticking rear center piston... GAWD!!!

[image: image7.thumb.jpeg.c35031795347e4be9f57868c61d8d307.jpeg]

[image: image9.thumb.jpeg.699bc347ac6b78e3177ef1cc5f8c591d.jpeg]

I put everything back together, and the brakes work fantastic now. Rear disc is still warm to the touch... warmer than the front discs. I wonder if running them hot cooked the seals in the calipers. Might have to rebuild those, AGAIN!!! ARGH!!! There are reports of Goldwings bursting into flames and other bikes blowing out their brake lines from overheating the discs and consequently the calipers and the hydraulics.

Moral of the story:  If you are rebuilding brakes on a VFR800 with neglected hydraulics, Secondary Master Cylinder REPLACEMENT should be a top priority! The SMC is the weak link when the brake system is neglected. I didn't know that. I rebuilt all calipers, seals and front master cylinder, but I took my chances with an old SMC because I was ignorant. If you are refurbishing a neglected VFR, do yourself a favor and buy a new SMC before it becomes NLA.

I had the same problem. I disassembled the rear caliper and replaced all the seals and cleaned the pistons. The center piston had what appeared to be water mixed with brake fluid. For bleeding I manually pushed the secondary master cylinder to bleed the center rear piston. Now that I’ve seen your photo it may be time to look into a new SMC. Good piece and photos. I should also think that the old SMC could be cleaned. I’ve had similar issues on a BMW Magura MC. The front brakes were locking up. I pulled the plastic reservoir off and found corrosion around the return orifice and cleaned it with a .009 guitar string. 

Marooncobra, 5/10/2020 at 4:29 PM

8200

Having now seen the cut away of the SMC I think when I do my brakes next I will strip the SMC down and throw it in my ultra sonic cleaner and hit it for a good ten minutes with a brake cleaner solvent.  It may fit in a peanut butter jar so I don’t waste too much solvent during the process. That hole is tiny.

Use a .009 (hi E guitar string)

Thanks for posting this @8200rpm , and for acknowledging that you do not know the source of the images.

For reference, the original diagram that you posted has been modified from my work - one of a sequence of illustrations that I designed to illustrate which master cylinders operate which pistons on the ST1300 - and to give some idea of the flow of fluid when each bleed valve is opened, and to explain the correct bleeding sequence.

The only difference between the st1300 and the VFR - as far as I can tell, is that the lines to the rear caliper are interchanged - ie the SMC acts on only the centre piston on the VFR, whereas it operates the two outer pistons on the ST1300.  My diagram has been modified by someone to reflect this.  

The original documents were published as a pdf file with full explanations and behaviours when trying to get rid of air.

That document is here and has much relevance for VFR users.  

https://www.st-owners.com/forums/threads/st1300-brake-maintenance-getting-all-the-air-out-of-the-brake-system.156754/

The photos of the cutaway SMC showing the oh-so-tiny compensation port / relief port - are also my work.

No problem - it all helps to spread a better understanding.  Just setting the record straight !

It is a few years down the line now - I created those images and the document 9 years ago (Feb 2017), but if you think that the full document would be useful to VFR owners, I can probably modify it quite easily to be accurate for the VFR rear brake plumbing.  Let me know.

jfheath 

May I humbly put my hand up, and say yes! If you have the time and inclination to make it relevant to vfr's, please do so.

@8200rpm what an excellent post, this really helped my understanding of the braking system which I now know I was not clear on! 

@jfheath If you are so inclined, that guide would be gratefully received. That diagram is really useful, thank you.