Rear brake hanging, center piston only

So about 4-5 weeks ago I replaced all the old lines on my '98 with a full set of Galfer lines, replaced the pads, etc and everything was fine. I've ridden the bike at least a dozen times since then and there have been no issues.

Today I go for a ride to a buddy's house and on the way there everything seems fine. Leave his house and I can tell something is wrong. I clutch in and the bike slows to a pretty rapid stop. Pop it up on the center stand and I can't spin the rear wheel at all. So, I bust out my trusty 8mm wrench and crack the outside bleeder just a touch. No change. Crack the inside one for the center piston and I get a small squirt of fluid and the rear frees up. I limp the bike 5 miles home, being very easy on the brakes but still have to stop once more to do the same thing.

Everything looks fine from a visual inspection, the fronts are not hanging at all, and it wasn't doing this before today. Anybody have any ideas of where I should start looking? I assume that I'm gonna spend next weekend bleeding the damn thing again, but any suggestions would be welcome.

Edit: further diagnosis shows that pumping the *rear* brake will increase the pressure on the center piston and it won't release. I'm leaning towards something being wrong in the secondary master or the PCV valve. Maybe I should start reading the de-linking threads. :smile-new:

 

There's no direct connection between the centre rear piston and the foot pedal that I can see. The only way the foot pedal has any effect on the centre rear is that it will activate the front centre pistons and the LH caliper will rotate and apply pressure to the secondary master cylinder. So unless you are moving the front wheel, the centre rear should not come on.

I think your diagnosis about the PCV being the culprit must be correct. It looks like a complex bit of hydraulic valving, maybe you have picked up a bit of grit in it that is stopping pressure bleeding back to the secondary master?

 

It looks like the PCV valve is that connection... The second to last step of the brake bleeding procedure (page 15-12 in the manual) basically shows bleeding from the PCV to the center piston, but the step before it is bleeding from the rear pedal to the PCV. I'll try flushing a ton of fluid all the way through and see what happens. A new PVC is $200 from PartZilla, so it's not the end of the world but is annoying.

 

The rear pedal when pumped will push fluid through the secondary master cylinder, which sends fluid through the PCV and on to the rear caliper center piston. If flushing fluid doesn't work, consider buying used, perhaps.

http://www.ebay.com/sch/i.html?_odk...rol&_nkw=vfr800+proportional+control&_sacat=0

 

Have you try to remove and rebuilt the pistons?
The rebuilt kit is fairly cheap.

 

It's possible the piston is sticking, but the fact that it retracts when the bleed valve is opened suggests it's operating ok.

 

Thanks for all the inputs. After more testing, I think it's the secondary master on the LF caliper. Working backwards from the rear, I know that if I open the center bleed on the rear caliper, the wheel spins. If I open the PCV bleeder when the piston is extended, I get a bit of fluid and the wheel turns free.

If I put pressure on the rear brake pedal and open the PVC bleeder, I have to *increase* pressure on the pedal and then something near the secondary master makes a squeaking noise and fluid flows. I'm wondering if somehow a piece of grit got in there and is allowing fluid past the piston when it shouldn't be. The "squeak" noise happens whenever I step on the rear pedal, and it definitely seems to be coming from the left front.

Rebuilding it doesn't look so hard, so hopefully that's somewhere to start.

 

Just wanted to finish this thread. It was the SMC. I didn't rebuild the bad one, I replaced it with one off another LF caliper. Everything is back to working as normal, and the rear brake pedal is less spongy than it used to be even though I bled that line a dozen times after the new lines. So, an overly spongy rear pedal may mean that your SMC is going south.

I also replaced the PCV since I could get one cheap and fast, and it made no difference. I did that first because I thought it would be faster and less mess. I was right on those counts, but it didn't matter. It was the secondary master. Careful road test this weekend, but everything seems right.

 

Thanks for closing off this post; it has been a good opportunity to learn more about the LBS. I'm glad you had a straightforward and positive outcome.

 

Glad you fixed it and thanks for posting the follow-up! :thumbsup:

 

Also, any plans to do a post-mortem on that SMC to figure out the root cause?

 

I will probably take it apart eventually... but after the Galfer line replacement and now this issue I'd rather not look at brakes for a while. I see you're in MA too, and with decent weather finally coming this weekend I'd much rather be riding than wrenching.