Rear Brake Bleed

[xsbean]

Okay I’m stumped here. Rear brakes working, pulled them off when I took the engine out. Had the fluid reservoir hanging upside down for some time. I’m assuming I introduced some air into the line as it would give me no pressure when I reassembled. So I tried… many times, to bleed the conventional way. No luck. I bought a vacuum pump, followed all the steps I could find on this site. I had absolutely no vacuum leaks while trying to bleed. Even taped the bleed nipple with thread tape to stop air entering. It wouldn’t even bleed. While vacuum pressure was still in the line I unbolted the the MC anticipating that maybe the brake pedal adjustment was compressing the MC, causing it not to allow fluid through the lines. It started to slowly bleed, with constant bubbles coming through the nipple. I refilled the reservoir nearly 5 times and no change. I closed the nipple and tried to get pressure in the lines by hand. No luck again. Does anyone have any idea what this could be? I have no leaks as far as I know and I would assume maybe the MC needs a rebuild but the fact it was working perfectly fine beforehand and also that upon disassembly it actually looks pretty clean and new in there. PLEASE HELP IM ABOUT TO LOSE MY MIND!

 

[jpdevol]

It is possible that the MC piston dried-out and is not sealing. How long has it been apart?

Assuming for now that the MC is good, air rises in the fluid, on difficult systems I'll bleed them at the highest fitting; probably on the banjo bolt atop the rear caliper. Crackit open while traveling brake pedal and close the bango fitting before the pedal reaches its travel limit. Tap along the line itself in between pumps to encourage the bubbles to coagulate in one spot (high point).

 

[halfmile]

I had the same bleeding problem once too. I had to unbolt the caliper and raise it way above the MC so the line was straight and the bleeder valve was at the highest point. Clamp the piston in so it does not pop out.

 

[gggGary]

yes if you haven't yet, rebuild the caliper.
the gggGary (n Jim ;^) hard brake method.
https://www.xs650.com/threads/jims-1980-sg-miss-september.50545/page-57#post-562380

 

[650Skull]

I unbolted the caliper, laid it on the ground and made sure the line was sloping down to the caliper, from the reservoir, so no air could get trapped in a high point....... made sure i had something between the pads to act as a stop.

 

[xsbean]

Thanks fellas, I’ll try all of the above. The MC wasn’t actually apart at all, neither was the calliper, so I’d assume they should work again. I did take both apart and clean etc. noticed some slight pitting in the piston but above the seal so shouldn’t affect sealing.

 

[xsbean]

Okay I have brake pressure! Although the lines probably still have air in them, I have pressure. I discovered that the MC piston was not fully retracting, it was subtle because the pedal always went straight to bottom until I pulled the bottom of MC out by hand to meet the pedal piston between pumps. Which allowed the fluid to enter the lines and build more pressure as I wound out the piston stop closer to the sized of the pads. Will probably try to bleed all the air out and then eventually rebuild the MC too as it’s binding for some reason.

 

[gggGary]

Caliper rebuild, the piston seal HAS to come out. A wad of glop builds up in the groove behind it keeping the piston from moving as it should. The piston then sticks to the seal instead of sliding past it, preventing it from moving out to the rotor.
Yes MC second port from reservoir to piston bore is near microscopic, EASILY plugged.

 

[gggGary]

I ended up having this same problem on Madness. Oops the gggGary bleed didn't fix the issue,
Take a look here https://www.xs650.com/threads/ggggarys-latest-xs-project-madness.28289/post-837553
for a possible (likely) answer to soft rear pedal issues after a brake overhaul.