No pedal resistance after brake bleeding

[bmwjbp]

I was bleeding the brakes on my 1979 R 100 RS. The bike is new to me and I wasn't sure how old the fluid was. I used a vacuum pump at the bleed screw to pull new fluid through the system. The front went okay and the existing fluid was a dark amber color. However on the rear the fluid was almost black. After bleeding enough to get clear fluid I have not resistance at the brake pedal. Is it possible that the black fluid was disintegrated seals in the master cylinder? How is the best way to test the master cylinder? Or do I have to take it off and tear it down? Thanks

[scjack]

The rear brake used on these models is sometimes difficult to bleed. I would try it again and then remove the cap from the resevoir for a few seconds to remove any vaccum lock. Put the cap back on and then see if you have any pedal resistance.

I ran into this on my 1978 R100S last summer and this method solved the problem.

If that does not work, you may need to replace the seals.

Good luck

[Friedle]

You've still got some air trapped in the rear braking system.

Go to www.airheads.org tech pages to see how to properly bleed the rear disc brake system on your bike. Then join the Airheads for access to lots more useful information about your bike along with a whole group of people willing to help you in a "hands on" fashion to learn how to maintain your bike as you add those miles to a fine motorcycle.

Ride it like you stole it

Michael Friedle

Airhead #46
Airheads Beemer Club Board of Directors

[jshuck]

try these:
1. is the bleed screw completely vertical? you may have to take off the caliper and move it slightly to get it vertical so the air can get to the top of the caliper.
2. tap on the caliper with a rubber mallet. This will work the air to the top.
3. Leave the brake pushrod pressing against the piston overnight by rubberbanding the lever to the handlebar or pulling the foot pedal down. This will work the air out.

[bmwjbp]

Thanks to all for the good advice. I also own a Moto Guzzi that I just finished this procedure on. Their shop manual specifically stated to remove caliper (from mount, not brake line) to bleed. The manual I have on the BMW didn't state that for the rear so I assumed I didn't need to do it. I have always bled my Triumph rear brake without removing the caliper, either. So I thought that the removal was just a Guzzi thing. I will try removing the caliper, getting the bleed screw verical and hope for the best. If that fails I wil try the rubber mallet trick -- except I will use it to hit myself in the head for all the trouble I have caused myself for such a simple maintenance item.

[manicmechanic]

I'm thinking if you follow the brake line routing from the master cylinder to the caliper you'll find a high point that has collected the air bubble, probably where it runs across the frame. So removing the caliper and making it the high point, then having a helper pushing the brake pedal firmly may help flush the bubble.

[dlearl476]

FWIW, I've never had good luck with MityVac and moto brakes. I much prefer SpeedBleeders and the old fashioned method. I do have a new pressurized bleeder I got for the P-cars and as soon as I can get a spare resevoir lid to make a fitting for the bikes, I'm gonna try that. (It blows instead of sucks)

[bmwjbp]

Yesterday I tried removing the rear caliper and keeping the bleed screw vertical during the bleeding. Unfortunately that didn't work. I concluded that it probably was a bad rear master cylinder. To test the hypothesis I disconnected the brake line from the master cylinder, covered the hole with my thumb and press the brake lever. I fully expected that there would be no pressure. To my surprise there was plenty of pressure. By pumping the lever and moving my thumb slightly I was able to get good pedal resistance. So I reconnected everything and bled the system using the pump the lever open the bleed screw method instead of the MityVac. I was able to get the pedal back to where it was before I started this adventure. However after one final bleed for "good luck", the pedal resistance was gone. I tried the procedure again starting with disconnecting the master cylinder, I wasn't able to get any pedal resistance. Out of desperation (or divine intervention) I wiggled the hose from the reservoir to the master cylinder. Surprise -- a parade of bubbles marched into the reservoir. I tried bleeding the master cylinder, got resistance, reconnected the brake line and re-bled the system. Now everything is fine.

It turns out that the hose from the reservoir to the master cylinder was the culprit the entire time. I think it is a little too long. A bend in it allowed an air bubble to block fluid from the reservoir to the master cylinder, hence the master cylinder could never build up pressure.

Thanks again to all who offered advice.