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Rear brake system problem

K

keelerb

Guest
1984 R100RT.

Cannot get any pressure in the rear brake system. New master cylinder, new line, no apparent leaks, I can bleed for hours (I have done so), there doesn't even seem to be any air coming out the bleed line, but I can't get ANY pressure up (enough to push fluid out the bleed line, yes, but that's it) - not even enough that I can't turn the rear wheel by hand with lever fully depressed.

I have reverse bled, vacuum-bled, "normally" bled, and speed-bleeder bled. I've pumped two full cans of brake fluid through the system.

I've always hated bleeding brakes but this is ridiculous.

Anybody ever run into this?

Thanks.
 
Bob, this all came about because rear brake lever travel had somehow become reduced to a very short distance from "off" to "full on." I removed the brake pedal and linkage, including the MC, cleaned and adjusted, and tried to refill and rebleed.

Now I have full lever travel all right, but no actuation of the pads!
 
Yes, have similar problem w/my '83 R100RT. Bleeding the system did not lead to a pressure build when pushing on the pedal. In my case, stroke after stroke of the bleeding process yielded only a tiny bit of fluid exiting the bleeder. I suspect mine is has bad "O" ring's or a mechanical disconnect somewhere between the pedal and the master cylinder piston. I'm off the road 'til I find time to pull and rebuild my rear master cylinder. You said yours was new. Are you sure it's in working condition? Let's keep each other informed on this one. florabunda
 
Right, next question(s):
When you're not getting pressure at the rear wheel caliper, how does the brake pedal/lever feel? Are you getting normal resistance there, less, or more? When you first press on it until it stops, does it feel like it's stopping due to fluid resistance, or does it just run out of travel after a steady "swoosh". Does it return freely, or do you feel like it's a little slower coming back than normal?
Also, when you're able to rotate the rear wheel, do you feel some initial resistance that steadily decreases?
 
I am getting little (if anything) beyond spring pressure (the one inside the MC and the one that connects the linkage to the frame). Don't really feel fluid pressure at all - very little anyway.

Depressing the pedal fully will add a little "drag" to the rear wheel, but that's it - I can still turn it by hand, easily.

Pedal returns to starting position readily, pulled up by those same two springs...

Maddening!
 
PS Florabunda, MC seems to work normally. So did the one it replaced, although I had that one on and off so many times I finally buggered the threads on the "working" end - retapped but I was getting leakage under pressure (I had been tying the lever down over nights as part of bleed process and would find a puddle in the morning....) Linkage works fine, god knows, I am now an expert on removing and reinstalling that....!
 
I'm wondering if you don't have a stuck piston in the caliper. Does the effort to turn rear wheel change when you try to apply brake? Seems contrary to sympton. Sounds more like ther is a fliud bypass, internal leak, in the master cyl. It's possible that a seal could tear, or twist if not assembled with a little care. If you have truely bled all air, it comes down to master. I'm assuming that the new line is a real brake line, not a substitute like fuel line. I know! But, I have seen some strange things, 5mm rubber fuel line about 6" dia, after a diesel to gas conversion.
 
Skiteach, anything is possible but I don't see how a stuck piston in the caliper would affect ability to build pressure in the system? - the pressure might not DO anything if the piston wouldn't move, but the pressure would still build.

Lines replaced were the reservoir to MC line (cloth covered rubber-like substance, used new BMW replacement part) and the final "rubber" line to the caliper, replaced with a Bob's BMW s/s line.

I dunno, maybe I have to remove the caliper and position so bleed nozzle is highest vertical point....but it isn't far from there when installed.
 
Exactly. If you follow the routing of the brake line you'll see that it runs at a level above the caliper as it crosses the swingarm. So remove the caliper and hole it higher than the swingarm, and have a helper pump the pedal and keep the reservoir full.
 
Thanks, I'll give it a shot. I didn't need to do that when I installed the S/S line, but maybe I got lucky.... will report results once I've tried....
 
Bleeding Brake

If you bleed with the caliper off of the disk, put something a little thicker than the disc between the pads. You don't want to blow the pistons out.

George Ryals
 
Skiteach, anything is possible but I don't see how a stuck piston in the caliper would affect ability to build pressure in the system? - the pressure might not DO anything if the piston wouldn't move, but the pressure would still build.


As I thought about it, I should have deleted that line. A stuck piston would bleed and probably give good pedal, short pedal travel and when/if piston moved would not (usually) allow wheel to turn freely. Sorry for that. My excuse is that I was celebrating the sale of my old truck-after 8 mos of trying- 01 F250 with plow, for more than my rock bottom price! Now I can ski Powder Mtn Utah, late Jan!:brad
 
moving the rear wheel is not part of the problem... you need to remove the calipar and invert it so the bleeder is the highest point, put a flat wench inbetween the pads while you bleed then reinstall and check brake function...
 
Rear Brake Problem

Yours would not be the first defective new master cylinder. You can test it by removing the hard line from the master cylinder and installing a bleeder in it's place. They are conveniently the same size and flare type. Bleed the master just as you normally would a brake system. After a stroke or two, with the bleeder closed, the pedal should go rock solid. If it passes this test, press the lever down a bit while the bleeder is open, then close it without releasing the lever. Press down again from this position. Again the lever should be rock solid. Repeat this routine until you've reached the bottom of the stroke. Unless the pedal will hold pressure at each step, you have a bad master cylinder.

If the master cylinder is OK, you can use the same technique to test the lines and hose to test them.
 
Ding, ding, we have a winner! Elevating the caliper (with bleed nozzle uppermost) made all the difference in the world. Two bleeds and I feel pressure, and it's like a "normal" bleed thereafter. I have it sitting with pedal tied down, I'll leave it like that for the day, bleed, do it again overnight, and I predict I will have normal braking tomorrow.

Florabunda, give it a try!

Thanks for all your help folks.
 
We're getting there then. If the master cylinder was bad, usually fluid would get past the piston and slow down the brake lever when it's trying to return.

A stuck or sticking caliper would resist and you'd feel pressure at the point of friction once you bled the air out. (Ever have one freeze? The pads won't retract and continue to rub the rotor after braking. The fluid heats up and expands, eventually locking the wheel and making the lever as firm as a footpeg.)

I'd try what tghsmith is recommending. Air trapped in the caliper between the bleed screw and the piston would let fluid out the bleed screw all day, and then just compress when you apply the brake without really stopping the rear wheel.
 
Thanks Bob. I seem to be on the right path now. Wouldn't have believed how profound a difference this made if I hadn't tried it.
 
Well, better late than wrong, as I was with my Gilda Radner reference.
Emily Latella said "Nevermind", Rosanne always said "It just goes to show".

Glad you're back on the road!
 
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