Topic: Interesting Rear Brake Failure

[ofreen]

It isn't often something goes wrong with my '75 750F, but today the rear brake gave out.  I noticed the pedal travel was excessive as I came to a stop.  I looked down and saw brake fluid dripping under the rear master cylinder.  I took it apart when I got home.  I figured that the master cylinder had developed a leak - understandable since it is 31 years old and has never been rebuilt. 

What I found wasn't what I expected.  The brake pedal linkage connects to a pushrod that actuates the master cylinder.  There is a rubber boot that covers the top portion of the pushrod and seals around the bottom of the master cylinder.  When I pushed the pin out that links the pushrod to the rest of the linkage, the pushrod fell free of the master cylinder (that shouldn't happen) and the boot popped off the cylinder.  The boot was completely full of rusty water about the consistency of mud.  The circlip that keeps the pushrod captured inside the master cylinder had rusted away, finally breaking in half and allowing the pushrod and piston to drop out of the cylinder.

The only thing harmed was the circlip.  There was a little corrosion on the pushrod, but it cleaned up fine.  All the rubber parts are in excellent shape and so is the cylinder bore.  I cleaned everything up, found another circlip. and put it all back together.  No leaks.

I suggest if you have an F model and you haven't had that area apart in awhile, it might be a good idea to check inside that boot, especially if you ride in the rain a lot.

[Bob Wessner]

Just curious, how long had it been since the brake system in question had been drained and the brake fluid replaced? It absorbs moisture and this could have been the source of your problem. Here's a pretty good explanation of what happens and why.

http://www.shotimes.com/SHO3brakefluid.html

Another interesting piece, slightly different perspective, but the end result is the same, fluid needs to be drained and replaced periodically.

http://www.babcox.com/editorial/bf/bf50412.htm

[Steve F]

Yeah, the same thing happened on my '76 750F, but the circlip didn't break.  Water gets into the boot and rusts everything.  My master cyl. was fine too, and just needed to be put back together and then I bled the system.  I was in there to rebuild it anyway and found the water.  The water never got past the plunger seal, and the fluid wasn't contaminated.  I then thought it was time to make all the "exposed parts" out of stainless steel.  Still using the same circlip though.

[ofreen]

Just curious, how long had it been since the brake system in question had been drained and the brake fluid replaced? It absorbs moisture and this could have been the source of your problem.

I change the brake fluid in all my bikes every year.  The inside of the master cylinder is fine and all the interior components were clean with none of the white crud you get when moisture gets in the brake fluid.  This was all on the outside of the cylinder inside the boot.  It is one of those out of sight out of mind things, so I gave the heads up.

[ofreen]

  I then thought it was time to make all the "exposed parts" out of stainless steel. 

I like the stainless steel idea.  Maybe I'll do that sometime.  In the meantime, I applied a film of brake grease on the pushrod, and put a little RTV on the inside lips of the boot.

[jaknight]

HEY THERE BOB------->
   Very interesting web links. I thought the second link was very interesting.

   It looks to me like some good stainless steel lines are going to be in me and my bike's future!!

   ~ ~ ~ jaknight ~ ~ ~