Rear brake issue

[LAB3]

This one has me stumped. About a week ago my rear brake just kinda decided it didn't want to work right any more. The fluid level in the reservoir was fine and there's no apparent leaks anywhere in the system. Decided to just wait it out until my new set of tires arrived since I needed to remove the saddlebags to get to the caliper. This kept my away from the rally since I didn't care to run the mountain roads any more than I needed to. Current at Smitty's in Tellico Plains.

Well yesterday was the day and I dug into it. Since I was also needing new pads I put them on and as mentioned there's no apparent leaks. Pulled the caliper off to get it up a bit higher and properly bleed the system just in case some air made it in there. A couple of tiny bubbles but no big deal BUT here's the puzzling part.

With the caliper removed to bleed the system I used a C-clamp to keep the caliper piston from pushing out, had a nice firm pedal so I put things back together. After putting the caliper back into position I get almost ZERO pressure to the pads, it'll slow the bike down at 5mph but it's a long way from stopping it, the pedal travels all the way down unlike it does with the C-clamp in place which gives you what you'd expect, about 1/2" of prdal travel and that's it.

Anybody have any ideas? This is an odd one for sure!

[Socer4m]

Howdy Lab,,,,check that brake fluid level is good in ther rear master ,I am guessing do a rear brake bleed and hoping that will get the air out of the system,,,sounds like air as the pedal travel is waay too much. Check the fluid level again and check to see if pads arent hung up,,,keep us informed. Mike in S.DIego and Pa

[cajun31]

Could be several things. Do you still have original lines on the bike? If so they could be breaking down. When was the last time the brake fluid was changed? Crud builds up quickly. When you had the caliper free did you notice if the piston was actually moving? Be me I would probably just pursue a complete and thorough go thru on the entire system. Disassemble as much as possible. Check the lines to make sure they have no impediments or blockage. Which to me is what it sounds like is going on. You would still have a firm pedal but no pressure to the piston in the caliper. Even with new lines if crud got down in the line you could have the same symptoms. Flushing the entire system would at least give you some indication of that and a refresh is a good thin anyway.

[DiverRay]

I agree with cajun31! If you still have the original brake lines, replace them. I had lines built at a local shop that does hydraulic lines for equipment. They have the correct ends and all, done in one day for the price of ordering something and waiting.

[LAB3]

As mentioned, there seems to be good pressure with a C-clamp on the piston but not when I put the caliper back into it's working position. If there a problem with anything mentioned above such as degraded lines, old fluid or leaks it shouldn't work with the caliper in either position!

Yes, the piston moves freely. The line was replaced with a stainless one about six months ago and the master cylinder was rebuilt two years ago. No, I didn't reuse the old brake fluid so it's also been in there about six months.

Let's try to stick to the original question here instead of reiterating typical brake maintenance, it's been covered.

[cajun31]

Your original post did not answer any of our original questions. The reason I asked them. On the rear master there is also a small adjustment that can be made on the brake rod going up into the master. If that is adjusted correctly then all that is left is to bleed the system better. The only thing that is left would be crud and that is generally worse, as you know, when the bike sits up and still for longer periods of time. You, being on the move all the time probably don't have that problem. These brake systems were never the best to begin with.

[bikerphil]

Maybe the rotor is warped or loose. When the wheel spins, it forces the piston back in too much, making the pedal need to travel further than with the stationary C-clamp. Just thinking out loud.

[DEEBS11]

I think Cajun31 hit the nail on the head. It's the length of travel. The adjustment of the length of the brake rod that pushes the master cylinder piston up into the bore. When you used the clamp you shortened the hydraulic distance needed to have solid pressure. When on the bike, the distance was increased and just out of reach of that travel. The pads wore until the present adjustment could not reach anymore.

[Radioguylogs]

I think Cajun31 hit the nail on the head. It's the length of travel. The adjustment of the length of the brake rod that pushes the master cylinder piston up into the bore. When you used the clamp you shortened the hydraulic distance needed to have solid pressure. When on the bike, the distance was increased and just out of reach of that travel. The pads wore until the present adjustment could not reach anymore.
}[/ATTACH]

Seems reasonable, but the new pads should push the system back into a working range?

[DiverRay]

And wear on the bushings and joints in the system will require and adjustment now and then. If I remember correctly, I had to adjust the rear s on my then new '78 twice in 32K miles to keep the brakes working like new.

[cajun31]

Seems reasonable, but the new pads should push the system back into a working range?

You would think that everything would still be within tolerance with new pads. Who knows for sure without being there to look at it. Clocking as many miles as he does who knows what else has worn out of tolerance. At least we have thrown a few good ideas his way and hopefully he can sort it out and get it fixed.

[DEEBS11]

Another thought: The bore could have some pitting in a certain area also where it would lose hydraulic pressure. A hone & rebuild kit would repair that if necessary.

[cajun31]

Just wanted to bump this thread to ask LAB what he found out.