Master cylinder woes

[Yikes]

  Now that I've gotten my '82 running fairly reliably, my next goal is to get an up-to-date inspection sticker on it.  I have this brake fluid leak at the handlebars that I think might be a problem at inspection time.
  There has been a slight leak from the master cylinder since I got the bike last fall and it seems to be getting worse with more use.  Up to now, I've been OK with wiping the fluid drip from the brake handle and adding brake fluid as the reservoir lowers, but a real solution is needed.  When the brake handle is pumped, a slight gushing sound comes from behind the brake lever and fluid can be seen exiting the cylinder where the brake lever pushes on the end of the piston.  Last weekend I pulled the perch assembly off to try and get at the master cylinder and piston to take at a look.  I got everything off OK, but I could not separate the master cylinder from the throttle body without applying more force to it than I was willing at the time.  (It was in the 70's and I was feeling kinda stupid for working on the bike instead of using this freakishly warm February day to be riding. I was also concerned that I'd totally disable the brake if I tried too hard to disassemble it and I decided that a leaky, but functioning front brake is better than no front brake.)
  So I'm wondering where to go from here. I'm guessing the easiest fix would be to get a good used throttle body/master assembly.  Would it be better to try harder to crack the bond between the master cylinder throttle body and replace the piston and/or seals.  Is there a rebuild kit that would have replacements for the wearing parts?  All advice, anecdotes and admonishments appreciated.
  Looking forward to warmer weather!

[not-so-fast-ed]

Me too.
Very small drop or two after a ride.
Gotta be a rebuild kit with "O" rings, diaphragm, seal etc.
 [smiley=undecided.gif]

[Rob Valdez 79 R65]

I am having trouble imagining where the leak is coming from.  Between the casting and the reservoir?  
If so, and you have a rectangular reservoir (as opposed to the early, round ones), then there should be a o-ring between these two that is replaceable.

I don't know.  I have only rebuilt a round-reservoir model.  There are differences.

I do know I never thought I would get the two halves of the master cylinder apart!  (see pictures)


click for more photos

[Bob_Roller]

With your difficulty getting the mastercylinder out of the throttle assembly, did you remove the two , allen head fasteners that secure the m/c to the throttle casting ? They can be missed easily, if you're not familar with it.

If so , there's a good chance you've got corrosion in there.

If you can do this, there is a snap-ring that holds the piston and seal assembly into the m/c itself, if you can get the snap-ring off you should be able to remove the 'innards' from the m/c.

Don't be too surprised if you find corrosion damage inside the m/c bore.

If you get lucky, and all you need is the normal wear parts, they are readily available.

[Ed Miller]

My piston and all that technical stuff came out as one piece, and BMW wouldn't just sell me new seals and stuff, but only a whole new piston assembly.  I have to admit it was pretty easy.  I had no corrosion nor wear in my bore, but only had to clean out solidified gunk.  Mine is, I think, an ATE like my calipers.

They will need to know the size in mm of your master cylinder.  The number is cast into the body someplace.

[Rob Valdez 79 R65]

My piston and all that technical stuff came out as one piece, and BMW wouldn't just sell me new seals and stuff, but only a whole new piston assembly.

like this?

I have to admit it was pretty easy.  I had no corrosion nor wear in my bore, but only had to clean out solidified gunk.  Mine is, I think, an ATE like my calipers.

They will need to know the size in mm of your master cylinder.  The number is cast into the body someplace.

On my round-type, it is cast into the part that faces the handlebar.  You may have to remove it to read it.

click for large image

You will also see the allen fasteners that Bob speaks of.

[Yikes]

I have the rectangular type reservoir, which is probably original to my ’82.  I checked the MC and it is stamped 12.  The leak is coming from the end of the piston assembly where the brake lever pushes on it, opposite the end where the hydraulic line connects.  I was hoping the problem was at the o-ring seal between the plastic reservoir and the metal master cylinder, but on close examination that joint was tight – so tight I was concerned about damaging it by trying to pull it apart.  Also, my Clymer manual says that the o-ring must be replaced every time the reservoir is removed, regardless of condition.  That seemed odd to me, but not having a spare and apparently not the source of the leak I decided to leave well enough alone and not separate the reservoir.  (Although I do eventually want to get it apart to give it a good cleaning.)
  I’m thinking the way to proceed is to try again to separate the cylinder from the throttle body and replace the piston assembly that Rob pictured.
  And not-so-fast-ed, be careful with that drippy lever.  It’s true what they say about brake fluid and finishes.  Left a couple of white stripes down my black fender.  One more thing for the to-do list.

[Justin B.]

There is also a phillips head screw securing the reservoir from the underneath side...  The rebuild kit is about $50 (highway robbery, in my opinion) but a new brake cylinder is well over $100.  The 12 on your cylinder means it's a 12mm bore, you will need to know that when you order a kit.

[tagordon]

That fit of the reseviour you said was tight. Well, tight don't mean it won't leak.
Cause when u pump that fluid it finds any weak spot. And it may only be a fraction of a mm. When ur removing or installing it you are fighting the resisitance of the entire part. There may be a very small part of the O ring that is damaged. I did the math & decided to start with the individual parts first.
The O ring on the reseviour is available seperately. I had same prob as you. Replaced the O ring there and on the plunger inside the housing. The one on the plunger inside the housing is not available seperately. At least not from BMW. I visited NAPA & they measured the old O ring. Matched it perfectly. Any shop that does hydraulic lines should be able to match it also. Then it developed a leak at the top cover. Put a straight edge to the cover & was it ever warpped. Thought about heating it and trying to press it flat.
Checked the price of the part & ordrered a new one. The top cover was the most expensive part @ $12.00 (if memory serves well)
I ordered the parts from maxbmwmotorcycles.com. I love dealing with them.

[Justin B.]

What we need to be able to come up with is the actual piston seals.  That way we could do a rebuild without having to buy a piston we don't need...

[Yikes]

Was too busy with family stuff this weekend to dive back into my brake problem.  At this time my leak is definitely coming from around the piston.  I had planned on biting the bullet and replacing the piston assembly, but first I will try find a seal that fits. I owe my local NAPA guy some business anyway.  I'm all for the cheap fix if at all possible.  I just need day when it's not quite nice enough for riding, but still tolerable for wrenching.  Hopefully the leak will not migrate as I stop the present leak.

[Bob_Roller]

It's been about 16 years since I had my brake master cylinder apart, but my vague memory says that the forward most seal, was a 'cup' shaped seal that didn't look like it was a user serviceable item.

Anybody have one apart to confirm this ?

[Justin B.]

Yes, it is cup-shaped, but so are all/most automotive master cylinders and the rebuild kits I have put in for them always come with just the rubber bits.  They are not that hard to install once you've done a few...

[tagordon]

Thats the word I was looking for: Piston.
Not plunger. Been doin too much work on the house.
And not enough on the bikes.

[Yikes]

Capital Cycle calls it a "plunger"  in its online catalog and I'm thinking I may be ordering one in the not-too-distant-future.