Topic: Rear master cylinder leak - rebuildable?

[jahmic]

I'm working on getting a 750 F3 back on the road, and experienced a brake failure a couple weeks ago that was discussed in another thread. The rear brake ended up slowly seizing on me, which I attributed to a bad caliper piston...the piston was pitted badly. I picked up a new piston and new seal, and started working on the caliper. When I reassembled the caliper and began to bleed the line I noticed brake fluid dripping from the bottom of the master cylinder.

I removed the master cylinder, and something extra-ordinary happened...the piston shot out the bottom of the MC as soon as it was removed...that clip was either left hanging or installed improperly and came loose. The piston itself will definitely need to be replaced; the shaft of the piston was actually breaking apart right above the rubber seal on the lower end ...should have taken a pic. At first I thought the lack of the clip allowed the washer to sit at an angle and wear into the shaft...but thinking back to how it is assembled, I'm not sure if that's correct or not.

I cleaned out the bore, and reassembled the MC. The piston was not frozen (obviously, since it just fell out), and I manually actuated that plastic diaphragm(?) inside the MC with the piston to confirm it functioned, which it does.

The MC is still leaking though, which I assume is why I am having a heck of a time bleeding the system and getting the brake to work. Used a syringe to backfill the line, draw out air, and still nothing. This is odd to me since the brakes actually worked prior to the failure.

Could the heat have destroyed my MC? Or can I get away with getting it to work with a rebuild kit? The leak doesn't seem bad...after 10 or so pumps of the brake I get a few drops from the bottom of the MC.

Can someone also confirm where the 'seal' is in the system, so I can better understand where the leak may be coming from? I'm assuming nothing should be getting past that plastic diaphragm, and that's the cause of my issues.

Sorry for the long post, just wanted to be as thorough as possible

Here is a pic of the MC for reference. Ignore the zip-ties...it's how I decided to hold things in place while setting that clip.



Hopefully I can get this fixed ASAP without too much cost...money is an issue these days and I'm hoping to get the bike on the road as a commuter since my car is soon to be on the mend.   I'll probably get flamed for saying this...but if I just need some patience and should keep attempting to bleed the line, please do say so. If the brakes can function with this small of a leak, I may ride it til I can source the parts I need to fix it properly. Just don't feel like chasing my tail if that's an impossible task.

[rbmgf7]

Trying to remember my F3 rear MC rebuild. So you're saying there was no clip retaining the piston? Have you owned the bike long enough to have worked on the rear brakes or is there P.O. involved? IMO, If there was no clip, it could've caused the piston to overextend and travel through the bore at an angle, leaving one side of the cups to not seat very well.

If there's not scoring in the bore, you should be fine.

Its about $20 for a kit so play it safe and rebuild it. Should come with a new clip.

[jahmic]

The clip is there, it was just dangling when I pulled things apart. Looked like someone removed it, then couldn't figure out how to reinstall it so they left it there hanging. Had the bike for about a month...and there is most definitely a PO involved...purchased it from a farmer that tried to fix it, he got it from an 80 yr old guy that had it sitting on his barn for who knows how long, his wife made him park it. This is my first time going through the brakes.

edit: Decided to pull the MC back apart...piston flew out again, so that ring clip is shot. I also saw some scoring in the bore, so it looks like I'll be replacing the entire MC. 

Argh....


Pics to follow...as well as me, cycling 100 miles this coming week to get to and from work in 95 degree weather. Hooray!

[jahmic]

Dead piston



Scored cylinder



Thanks gearbot for pointing out that I should check for scoring...didn't see anything initially, but went back in with an led pen light and was able to pick out those 2 faint marks further inside the MC.

[jahmic]

Progress.

I decided to pull everything back apart. Gave it a thorough re-cleaning and...NO leaks!

I'll be ordering a rebuild kit soon, but th MC seems useable. I traced the leak to a tiny pit on the primary cup that lined up with the score on the cylinder. Cleaned up the score a little, seated the primary cup, and I'm having a much easier time bleeding the system now.

I also pulled the ring clip off the shaft to inspect it. It was bent a tad and probably didn't seat properly. Got that straight and popped it back in. So far, so good.

[jahmic]

Got it sorted. Took all day yesterday to get most of Tue trapped air out of the line by backfilling with a syringe, then playing the waiting/pumping game. Weighted the pedal last night, and still had trouble this morning as I had no pressure and still some trapped air. The location of the MC in relation to the reservoir and bleed valve surely allows pockets to get trapped in the MC and line...even with the caliper hanging above the line, I still couldn't get the MC primed.

Went to harbor freight, picked up a mity vac...20min later the brakes are bled and I have zero leaks still.

Looks like I'm going to order a rebuild kit since the cylinder repair seems to have worked.

[jahmic]

OH, and FWIW, gearbot, you were dead on. I think the leak was in fact due to misalignment of one of the cups due to that actuator overextending without the clip in place. I remember after setting the clip the first time and pumping that something didn't feel right as I was getting a click at the bottom of the brake's extension right when I noticed the leak a second time. After repairing the clip there was a much more noticeable and consistent end of the travel in that rear brake.

Thanks again for the help.