R100S Front Brake/Master Cylinder No Pressure

[VintageBandit]

Hello I'm new to the forum and I have a few questions for you experts. My first question for you is related to a dual-disk front brake system.

I just brought my friends 78' R100S home today and have been tinkering with various things and noticed the front brake lever has no pressure.

I took the tank off the bike carefully and set it aside, then checked the reservoir and it had no fluid in it. I added new DOT 3 fluid to the reservoir and then replaced the cap. I actuated the lever about 30-40 times and got no pressure.

I decide to take the speed bleeders loose and actuate the lever, which did push fluid out of both bleeders. I tightened the bleeders down and still had no pressure in the lever. The reservoir was still full.

Neither brake cylinder is frozen and I'm not sure where to go from here to look for a cause/fix. Any help would be appreciated!


-Adam

 

[kstoo]

This is totally unscientific and it is late at night so excuse me. With the ATE under tank master cylinder I have had to "prime" the system by taking the lines off the master cylinder and holding a finger over the port. Something like you might do with a spray bottle that is not behaving well. I have been experimenting lately with a vacuum pump but I can't say yet if that is a solution. Out of desperation once I used a venturi sprayer to suck the fluid through the speed bleeder but that again is unscientific. Once you do get it flowing try loosing the master cylinder and propping it up so that any bubbles will rise toward the output port in the front.
If it has been empty for awhile you may have to disassemble everything and clean out the DOT 4 brake fluid jelly that has spontaneously generated from absorbing moisture from the atmosphere. Don't ask me how I know this because I let a brake system sit empty and open for too long.

 

[manicmechanic]

One other place where an air bubble can remain is at the brake light switch on the front of the master cylinder. The rest is as kstoo says, just might need a good cleaning.

 

[20774]

If the bubble forms at the front of the brake light switch, you should remove the master cylinder and tilt the rear of it up and continue the bleeding process.

Another thing you may be facing if the master cycliner was dry is rust along the cylinder bore. It will tear up the o-ring, leaving you with no pressure again in the near future. If the rust is light, a small bit of sanding might help. If it's bad enough, you can get it resleeved or maybe just have to get a new one.

 

[VintageBandit]

Thanks for all the excellent replys! I will try all of your suggestions this week after work and report on my findings.


Thanks for taking the time to help folks!

 

[8ninety8]

dual bleeding

If I remember correctly, this may be backwards?? you've go to tilt the fork so the that bleeding screw is at an angle that can release any trapped air to the top of the caliper. Also, as mentioned, raise the back of the master to allow any air to rise from the front of the master to a point where it can pass through to the resevoir or be pushed out into line.

I think you tilt the fork to the left if you bleeding the right caliper? Visa-versa for the left side. Other remarks??