Brake Overhaul - Rear Master Cylinder Leak question

[Guest]

I'm getting back to working on my GS550 and started with the brake overhaul.

Rear Brakes
I didn't get far on the rear brakes. I removed the brake pads and caliper. I had planned to use the rear master cylinder to push out the pistons but when I depressed the pedal, brake fluid started leaking from the boot on the rear master cylinder. I'm guessing a seal failed or something like that while the bike sat for several months. Here is where it is leaking:


I guess I should just drain the fluid and see if I can get my hands on an air compressor to get the pistons out of the caliper. Is the solution for the leak to order a new rear master cylinder piston & cup set (~ $28 from Part Shark) - I don't see any other relevant parts on the fiche for the rear master cylinder. I also plan to order the seals & boots for the caliper pistons.

Front Brakes
I was able to drain the fluid and disassemble the front caliper. I still need to order a pair of the Circlip Pliers to disassemble the front master cylinder. I haven't noticed any leaking there but do most people order a new piston & cup set for the front master cylinder as well (~ $50 from Part Shark)?

There was quite a bit of gunk in there but the caliper piston seems to be in good shape:

[scott]

I was told that I needed to inspect the master cylinder closer before ordering parts as it may not be rebuildable if the bore is pitted.

I removed the master cylinder from the bike. This piece seems to be in bad shape (push rod according to the fiche):


I had to wait until I got a pair of circlip pliers to get it apart further.

Here's a few closeups of the rod end:


Different light:


I'm not sure if that piece is crucial but it looks pretty corroded. I don't see any rubber on it so I wouldn't think it has any part in my leak. Should I replace it regardless?

It's hard to tell but is #4 on the PartShark fiche that entire rod (including the corroded piece in the pics)? It's labeled as "ROD, PUSH 69670-45001 (replaces 69670-45000)".


The inner part of the master cylinder doesn't seem too bad - is this the bore? At the very back, there is a bit of black something. I tried to get at it with a q-tip bit it didn't seem to come off.


The outer ring does seem a little rough though:


The piston/cup piece doesn't look terrible but I was assuming this was the piece responsible for the leak.


Any thoughts as to whether this is rebuildable? And if so what parts I should be replacing? I was thinking:

• #2 - PISTON & CUP SET 69600-45820 (replaces 69600-45810)
• #4 - ROD, PUSH 69670-45001 (replaces 69670-45000) - assuming this is the entire piece
• #6 - BOOT 69691-45001

[hampshirehog]

I would kill to find a rear MC in that nick over here on an unrestored bike! Just clean everything up and replace part #2 the piston and cup set. Your bore looks fine and they usually only leak because that rubber seal just gets a bit tired.

[chuck hahn]

The push rod isnt an issue..wire brush the crap off. Its the cups on the drive piston thats the issue...and the bore of course. If the bore is smooth and shiney then replace the innerds and button her up...good to go for the next 20 years or so if you keep up on regular fluid flushings.

[chuck hahn]

Better yet....if your waiting on the parts, then throw the pushrod in a can of vinegar for a few days..crust will dissolve and a simple wipe off will have it looking good again. The pitting on its of no issue either.

[Big T]

Agreed - rear MC is totally rebuildable

Front - no need for a new piston, just get the seals

[scott]

Thanks for the advice guys! That's great news.

I've got the push rod soaking in vinegar now. I'm still compiling my list of parts so I definitely have time to let it sit for a few days.

I now have a question about my front master cylinder (perhaps I should start a new thread due to my thread title, but anyway). I was cleaning the front master cylinder today on the bike and decided I better take it off the bike to be more thorough. During disassembly, I noticed that it looks like brake cleaner was leaking out around the boot of the piston/cup area. After reading on here, I was thinking I may have gotten some brake cleaner into the "air hole".

This is my master cylinder with 3 holes on the inside (other than the reservoir mounting screw holes).


My assumption is Hole A is the air hole, Hole B goes to the piston/cup set "fluid side" and Hole C goes to the brake lines. Or do I have Hole B and C backwards?

I was basically covering up the master cylinder with a towel and spraying around in there with brake cleaner (mostly in Hole B but a few times the nozzle piece came out of the hole). I was thinking there are two options:
1) I got some cleaner into Hole A, which would leak out the piston cup/boot area and would not indicate a problem.
2) It went down Hole B and leaked past the piston cup/set.

Since the fluid wasn't particularly "brown", I'm thinking it was option 1.

[scott]

The more I think about it, I'm pretty sure I just got some cleaner in Hole A and there's nothing to worry about.

[chuck hahn]

Hole C is the return pressure relief hole..itm lets the fluid back up and release the fluid pressure on the pistons. Take a small wire off of a wire brush and poke it out. take out the clip and pull the inners out and clean everything well and blow air down the holes as well.

[scott]

Man - the front master cylinder is a lot more difficult. First, I ripped the piston boot trying to get it out. I used some long needle-nose pliers to try it work it loose but it seems like it must be caught on something. There is still some in there.

I can see some brown goop in there so maybe I did have brake fluid leaking past the piston/cup after all. Or maybe it's just dirt that got past the boot.

I was able to get the circlip out with much difficulty. But how do I get the actual piston/cup out now? Shouldn't it pretty much pull/fall out once the circlip is out. It's really tight in there with not much room to work or grab on to things. I seem to be marring up this side of the bore but I assume it's the part of the bore on the other side of the cup that is important. Am I missing something?

[hampshirehog]

If you have the circlip out first try pushing the piston in and out real fast. Hopefully the spring will pop it out. Sometimes you do have to resort to brute force and ignorance though and grap it with mole grips.

[scott]

If you have the circlip out first try pushing the piston in and out real fast. Hopefully the spring will pop it out.

I gave this a try. The piston didn't really push in/out fast (quite a lot of resistance). In any event, it didn't fall out.

Sometimes you do have to resort to brute force and ignorance though and grap it with mole grips.

I resorted to this using the smallest vise grips I had. I eventually got it to move out some but it still hasn't come out! I'm taking a break before having another go at it. The metal is getting a bit beat up from this. When something is this difficult, I tend to think I missed something - maybe mine has an extra circlip on there ...

[hampshirehog]

Ok - now you're in to final but drastic territory. Heat the thing with a blowtorch and keep using the mole grips. It'll booger the paint (and remove the reservoir if you have the plastic type) but it does shift the thing. There is only one circlip.

[chuck hahn]

HEAT....WHAT!!!!! Just take a phillips screwdriver and go in the hole the banjo bolt goes in and tap it out the handle end..done.

[hampshirehog]

HEAT....WHAT!!!!! Just take a phillips screwdriver and go in the hole the banjo bolt goes in and tap it out the handle end..done.

Chuck have you got a screwdriver that goes round corners? Just kidding but obviously you can do that on the MCs that have an 'in-line' banjo but when they are at 90 degrees it won't work.