Puzzled with Rear Brake

[CK]

A month ago while riding, my rear brake completely failed. Checked the pads and decided that might be it and replaced them. No such luck. Next, I purchased a MC rebuild kit, cleaned out all the porting, re-filled the system, bled it, but it wouldn't build pressure. A friend looked at the MC and suggested I get a new one, so I purchased a used one off of ebay. The new (used) MC, assembled with the rebuild kit parts proved to be better than the old one - the plunger's stroke was much smoother, but still no pressure build-up. I assembled then disassembled several times. Finally, on my workbench with the whole assembly together, I got it working. It took a while to get the air out, but after repeated stroking of the MC and watching the bleeder valve with a piece of clear tubing connected to it, it appeared all the air was out and the caliper was moving as it should. No leaks anywhere. I re-installed the entire system back on the bike without disassembling it. At first, the pedal would fully deflect as before and frustrated, I quit and left it alone. I came back two hours later and Voila! - it was pressurizing and would stop the back tire when it was spun. Today, after nearly a month of not riding, I was going to go on a short trip. To seat the new pads, I decided to do a few checks with the brakes in the neighborhood before heading out. To my surprise, the brakes didn't pressurize sufficiently enough to stop the bike. Back in the garage, application of the brake will still stop the wheel from turning, but apparently its not where it needs to be. I apologize for the long post. but I'm at wits end what to do next. It seems that fully compressing the caliper to get it over the new pads causes the system to fail at building pressure. Any advice would be appreciated.

Thanks,

CK

[BA80]

Take the caliper apart, there may be enough crap built up on that piston that it's catching on the seal in there and hanging up acting like a spring and pulling the piston back in too far.

Really sounds like there is still air in there though. It can be tough to get out.

Cleaning out the caliper certainly can't hurt either way.

Also, check the hoses back there for bulges. If the inner lining has failed and not broken through the outer layers yet you should be able to see or feel a bulge in the hose with pressure on the brake pedal. I can act like a spring.

[DGXSER]

IF you have not cleaned the caliper whiel you were at it, you definitely should have. Do that first, make sure you get all the crap out of that slot the seal sits in.

But yes, it could easily be air still in there. Have you tried pumping the master cylinder repeatedly, does the brake get firmer?

[bikerphil]

Had a problem bleeding a rear brake on a member's bike once. He went thru the whole system and still couldn't get it to bleed. I noticed that his brake pedal was out of adjustment holding the MC plunger in too far not closing off the internal bypass hole. There should be about 7mm of thread exposed between the adjustment nuts on the plunger shaft. Proper adjustment is explained further in the service manual if needed. Hope you find the problem.

[planedick]

What model?

You failed to say what model your bike is. Some have a linking system which applies front brake also and the problem could be in the linking system. The rubber hoses which were stock then could be a problem now and should be replaced with a stainles braided covering type to eliminste swelling of the old hose causing loss of pressure.

Put your bike model and year in your signature so in the future when you have questions everyon can see what model you have. Makes it much faster responses and probably more usefull information.

[CK]

All, thanks for the quick replies and suggestions. The bike is a 78E model. I will disassemble the caliper this weekend and check it out for the items mentioned as well as the brake pedal adjustment. Thanks again - I'll update over the weekend.

CK
78E

[natemoen]

All, thanks for the quick replies and suggestions. The bike is a 78E model. I will disassemble the caliper this weekend and check it out for the items mentioned as well as the brake pedal adjustment. Thanks again - I'll update over the weekend.

CK
78E

Could very likely be the spooge hole as well. It is a little tiny hole in the master cylinder that allows the pressure to release. Since it is so small of a hole it quite often get clogged when sitting for any amount of time.

[redbone]

yes, does sound like a spooge hole problem. it is very hard to see on the rear brake mc. it really helps to have one of those mity vac brake vaccuums as well. i think the rear brakes are harder to bleed than the fronts.

[DGXSER]

His problem is the brakes not making pressure. The typical result of a plugged spooge hole is the brakes will not release. The purpose of the spooge hole is a return vent, to allow the fluid to return to the MC reservior.

[natemoen]

His problem is the brakes not making pressure. The typical result of a plugged spooge hole is the brakes will not release. The purpose of the spooge hole is a return vent, to allow the fluid to return to the MC reservior.

Wow, I am mixing up posts, I swear this was the one where the caliper started smoking.

[CaptonZap]

Wow, I am mixing up posts, I swear this was the one where the caliper started smoking.

What are YOU smoking? CZ

[BA80]

What are YOU smoking? CZ

Must be Rocky Mountain High. .............WAIT that's Wisconsin..........bad cheese curds......

[CK]

OK, problem solved. There was a little more air that I got out of the system, but the real fix turned out to be a total readjustment of the pedal and MC stroke. I adjusted it to the values in the shop manual and that did it. Can now skid the tire if I want.

Thanks to all for your help. It's good to know so many experts are just a few computer post away.

CK