Help needed with linked brake bleeding on a 5th gen.

My SMC had developed a small fluid weep from around the boot/clevis area, not major but enough to dribble down the bracket and a few droplets on the leading edge of the fairing. I happened to have a spare piston assembly, so I pulled the circlip out, removed the old parts, visually checked the bore for marks and found none, and proceeded to reinstall the new/old piston, circlip, boot and clevis. So far so good, but bleeding hasn't been fun.

I have followed the manual, and bled the front centres first, then the PCV, the rear centre and finally the rear outer. Problem is the SMC does not seem to hold any pressure; when the bike is parked I can stroke the SMC fully with little resistance. With the pedal pressed, there is SMC pressure. While riding, the pedal itself is inconsistent, will feel firm on one application, then needs a little pump up for the next.

Is this a bleeding problem? I have been working on my own so not able to simultaneously press the pedal, SMC and bleeder (although I do have a speed-bleeder which has been good).

Suggestions welcome.

I would say 100% yes to air. 

I was helping a fellow 5 Gen owner with his brakes because the rear kept locking up, although kind of the opposite of your issue, there might be a common culprit, or one to at least look into. Inside the SMC there's a check valve and we found that his was completely clogged and ultimately he ended up just buying a new SMC, as cleaning it didn't seem to resolve the problem. I took some time to look at this check valve when dealing with another SMC for a different friend and found that if the orifices that the check valve is located get clogged as well as the check valve, bleeding the brakes and getting them to work properly can become a chore!

Thanks chaps. Hopefully I don't need to clean out the one-way valves but hey-ho, will do it if I must, 

I've convinced myself I just have a trapped air problem. I can pump the pedal and get a firm feel, but when released I can see the fluid level in the reservoir rise up, and the pedal is spongy again. Sounds like I'm compressing air to get a firm feel, and the air is then expanding and pushing fluid back out of the compensation port. I found a great post from Cogswell that gives a detailed procedure that I will try today.

Spent many "happy" hours in the Mancave today, chasing brake fluid around the many parts of the VFR. I'm pleased to report that all is back to normal, with good brake action and feel. The key I believe was the PCV bleed sequence, not easy to do solo but not difficult with a willing helper; turns out my wife was superb in this role, suprisingly trainable and obedient. And not on this forum. With an extra pair of hands, I did not use a speed bleeder. 

For those who pass this way in search of wisdom:

With the left caliper dismounted and angled to allow air to rise through the rear hose;

1. Pressurise the SMC with the foot pedal

2. Crack open the PCV bleed at the rear of the fuel tank

3. Depress the SMC piston with your hand and hold it in, some fluid will escape the bleed

4. Close the PCV bleed

5. Press the foot pedal to refill the SMC and push the piston back out

6. Repeat as needed

It is also possible the bleed sequence could be incorrect in the service manual. This states that the rear system should be bled right front centre, left front centre, PCV, rear centre, then rear outer. The ST1300 (which also uses a similar linked brake setup) originally used the same sequence but this was updated in later editions, so that the PCV is first, then right front centre, left front centre and then the rears. The 6th gen VFR uses a similar sequence (but lacking the right front centre, as this is not part of the rear system on this model). 

Cogswell's post from last year was mighty helpful. 

While not directly to the issue in the original post, I would like to comment with an alert on the return valve problem mentioned above.

A couple of summers ago we found a crashed Ducati ST2 at a low price in the town north of ours.  We grabbed our trailer and headed over to take a look.

The left side bodywork was trashed.  The lady doing the selling was the wife of the owner.  

Brembo master cylinders apparently commonly have this return valve problem.  So, when the brake is applied, pressure in the system increases but can't decrease and release the pistons.

The owner had put the battery back in the bike after a winter rest and gone for a test ride down the residential street.  No gear.

In two and a half blocks later, the front brakes unexpectedly locked up solid and threw the owner onto his head.

Months in the hospital and more months in rehab, he survived, barely.

I bought the bike, test-rode it with a trace of disbelief, and sure enough, locko-the-brakeo, and down it went.  I was okay apart from wounded pride and a bump on the knee, but I had to walk back home to get a wrench to open the bleed valve to release the brake enough to roll the bike.

I considered rebuilding the master, but a new one seemed much safer, and that was what was done.

As for the whole bike, I miraculously found a body set on Ebay with opposite-side damage for only $450 shipped- from Italy!  The bike went back together fine and I enjoyed riding it for a while before selling it to move on to the next project.

21 minutes ago, Cherryriver

front brakes unexpectedly locked up solid

What what? Any idea how that happened? I've seen pads installed wrong create problems, but nothing like this. I'd rock an ST2...or ST3. 

IMHO many bikes and even cars suffer from brake system neglect.  I change out fluid on everything 2 wheels or 4 every 2 years whether it's driven much or not.  My commuter car is a 1997 model Toyota. It has the original master and rear wheel cylinders and front calipers, now has 250,000 + miles.  No leaks, no spongy pedal, still has good braking performance and in the snow / ice I've had the ABS cycle as it should.  I probably should change out the hoses due to age, will get to that at the next brake service.  Those degrade from the inside and can become constricted, possibly leading to the locking behavior, but it could be due to other things too.  I don't think it's a stretch to guess that many bikes that are now well in to their 20's have very low miles (at least relative to similar age cars)  and have never had a brake fluid service since they left the factory.  Few miles ridden so still plenty of pad left so the owner figures that all is good.  With the complexity of the linked system on 5th and 6th gens (and Blackbirds while I'm at it), clean fluid is even more crucial for proper operation.  As always, YMMV. 

3 minutes ago, Cogswell

IMHO many bikes and even cars suffer from brake system neglect.  I change out fluid on everything 2 wheels or 4 every 2 years whether it's driven much

You  are right on the money there.  I do the same for my bikes. Well, on cars, I  am embarrassed to say, I am neglecting it. Now, next spring I will plan in a fluid change for the cage.

10 hours ago, bmart

What what? Any idea how that happened? I've seen pads installed wrong create problems, but nothing like this. I'd rock an ST2...or ST3. 

I'm more familiar with brakes and hydraulics now than I wanted to be. All master cylinders have a main intake port and a tiny compensation port. The compensation port allows excess pressure to bleed back to the master after application of brake or clutch, and is very easy to block with debris if maintenance is lax. When that happens the hydraulic pressure can build until the brakes start to bind or your clutch starts to slip. As the brakes bind they will generate more heat and that will cause more fluid expansion and that will jam the brakes on harder. In the case of the linked brakes on VFRs, CBR1100's and ST1100/1300s the rear brake piston driven off the SMC is often the point of failure and gets jammed on, either because of a blocked port or due to a seized SMC piston.

Don't skimp on the maintenance boys and girls!

Interesting stuff. Thanks, all. 

I believe in monitoring and doing maintenance when it is needed. I don't subscribe to overmaintaining for a false sense of doing good. 🙂 We see human error a lot at the track from folks doing this sort of thing. 

I've worked on a lot of bikes whose fluid hasn't been changed for years.... to fix requires money and effort.  Given the maintenance schedule normally says 2 years flush and bleed, I say get the thing into shape, then a quick squirt every year will save you a lot of time and money in the end.  Ounce of prevention as they say.

Can't agree more. I collect my new to me 01 VFR800 this Saturday and seeing how it's only done 8900 miles from new (documented), on my list of Things To Do is overhaul the brakes. New braided hoses (already purchased, £130 off at the bike show in Nov), caliper strip and replace seals etc then lovely new fluid. I'll probably replace the pads too.