Why is the Rear Brake Such a Pain to Bleed?

[IanDMacDonald]

I literally spent all evening on the rear brake, with no success. I reverse-bled the front brake under ten minutes, and the lever is stiff. I spent over two hours on here reading all of the threads regarding this issue.

Last month I broke down both master cylinders and re-built them. I did not re-build the rear with new parts, but the piston and seals looked fine. Cleaned them all out, and they seemed fine.

I read a few times of Greg mentioning the pedal adjustment, so I made sure that when I insert that rod into the rubber boot, that it is just barely touching the piston. However, for this to occur, it is less than 7mm between the nuts.

As far as bleeding the rear, I tried a few things:

First, I pushed fluid from the bleed valve to the M/C with the banjo at the M/C loose. This shot fluid out that opening. I then tightened that banjo and tried it again. No real fluid movement into the M/C.

Second, I emptied the M/C almost to the bottom, and pushed fluid from the bleed valve to the M/C. This allowed the M/C to fill. I tightened the bleed valve and continued to pump the brake arm. No real noticeable difference in brake pedal feel.

Third, I bought a vacuum bleeder from Harbor Freight and tried that. That thing was a p.o.s.!!! I'll be returning that tomorrow.

I ordered a rear master cylinder rebuild kit just in-case there are any issues with the seals, etc. I think what I'll do is bring the whole rear brake system inside and see if I can bleed it off the bike, then place it back on the bike and see how much more bleeding I have to do.

Any suggestions or advice would be greatly appreciated.

[Schming]

Hi Ian, sometimes when I use my HF vacuum bleeder I have to smear grease around the bleeder threads to seal of the air infiltration.
Also if you decide to bleed the system off the bike use a spacer, in the caliper, a smidge wider than the disc rotor to allow easier installation.

HTH GL

[natemoen]

The rear is a pain because of the vertical orientation of the master's piston. There is a little area at the top where it can hold a pocket of air.

Easiest way to bleed it is to unbolt the master a tip it.

Also did you bench bleed the master before you put it on? Not necessary, but it does make it easier.

[DGXSER]

In the past I have always had to use a vacuum system on the rear MC. Mity-Vac is what I have used. Even then I had to pull it off the bike to get it to work.

I just bled the brakes on the SG, and here is how I got it to work very quick and simple on the rear. I may have just got REALLY lucky, but it worked for me.

1 Pop the black plastic inner fender loose on the MC side, it just slides over a coupe tabs on the frame. You'll need to pry it over off the tabs. This lets you pull it back toward the rear wheel to access the banjo on the MC.

2 Pull the banjo off the rear MC, and "bench bleed" the rear MC in place. If your not familiar with the procedure, with the banjo out, put your finger or thumb over the opening the banjo goes into. Now pump the pedal until you feel pressure on your finger. Quickly pop your finger off and back on. Repeat this until the brake fluid is forced past your finger, you will not be able to hold back the fluid, but keep your finger pushed on tight. And yes, you will want a rag around to clean up the fluid that shoots out.

3 Have the banjo bolt on the line with the crush washer behind the line and in front of the line in place. Quickly remove your finger and install the banjo bolt. Tighten it up.

4 Bleed through the bleed nipple on the caliper like normal. I start by opening the bleeder, push down on the pedal, close the bleeder. Repeat until fluid comes out the bleeder.

5 Then I put a hose on the bleeder that goes to a container with a little brake fluid covering the other end of the hose. Now push the pedal till you get a little pressure, open the bleeder while pushing on the pedal till it bottoms out. Close the bleeder. Release the pedal. Repeat till good hard pedal.

Like I said, I have struggled with this system as well, this is what worked for me...this time.

[IanDMacDonald]

Thank you Schming, Nate and Don for the tips. I will probs not get to this till late tonight, but it sounds encouraging.

[crazy steve]

If you're using a vacuum-type bleeder, sealing the bleed screw threads to the caliper body is critical. Great place for some Teflon plumbers tape...

[D0wn5h1ft]

Speed Bleeder

Invest in a speed bleeder. Best money I ever spent on my '78.

http://www.xs11.com/forum/showthread.php?t=10116

http://www.mikesxs.net/product/29-0503.html

The guy in the thread I posted said that that one will work. Wish I would have seen his 2 days ago when I was ordering parts. Could have picked one up for you to save on the shipping.

[IanDMacDonald]

Speedbleeders work fine on a pressurized system. They're absolutely useless on a bone-dry system.

[bikerphil]

Hopefully you put the MC plunger back in correctly with the rubber cup facing the banjo.

Which HF bleeder did you get that didn't work?

[IanDMacDonald]

Hey Phil- I believe I installed the MC parts correctly. However, I will have another go at it when the new MC rebuild kit arrives. I bought the HF copy-cat of the Mity Vac. The seal on the cup was bad, allowing air to escape. Turns out after just watching a video, I was using it wrong anyways. I'll go back to HF in the morning and try another one. I also bought some Speed Bleeders for the end after all is done for down the road convenience.

[BA80]

I read a few times of Greg mentioning the pedal adjustment, so I made sure that when I insert that rod into the rubber boot, that it is just barely touching the piston. However, for this to occur, it is less than 7mm between the nuts.

Be sure the spooge hole in the master cylinder is not clogged and that the piston is coming all the way back to rest. It's easy to get the push rod a touch too tight especially on a fresh rebuild. Sometimes the piston will stick a little and keep the spooge covered.

Get the system full of fluid and pump the pedal slowly with your finger over the bleeder. You should get fresh fluid with every pump and your finger should keep air from being drawn back in when you release. If not the cylinder isn't "recharging" itself from the reservoir when released as it should.

[IanDMacDonald]

I'll attack this all over again tomorrow morning. I also read in MP'sthread about alignment marks of the brake pedal. While trying to bleed the system, I had the pedal on at a random point, pointing down at an angle. I will re-install the brake pedal correctly, and follow the advice from above and hopefully see results.

[IanDMacDonald]

I just read in so many different forums on how poorly the Mity Vac and other vacuum bleeders perform. I also read that it's possible to pull air past the seals throughout the system, as well as the cup in the M/C. I'll save my money and try Don's procedure in the morning. But, it appears what Schming and Steve said is "critical" when using the vacuum bleeders: threads must be sealed.

[crazy steve]

I figured I'd wasted my money on my Mityvac... until I sealed the bleed screws. It really does make a huge difference....

[IanDMacDonald]

DAMN YOU STEVE!!! Now I have to goto HF again and try the Mity Vac. I was dead-set on trying Don's Method.