Mystified - Front GL Master fixed, I thought

[Guest]

I rebuilt an '80 front master cylinder with OEM parts (piston, spring and cup), an '80 GL with the canted reservoir. This fluid reservoir stays horizontal with brake fluid even though the handlebar angles down. It's leaking brake fluid out the back, where the brake lever pushes on the piston. Rebuilds are not new to me, I've done it before on my '82 square master cylinder, but this one is that round cap, 79-80 model front master cylinder. During the rebuild, the piston bore looked good, and I used the long throat tool to get out the metal circlip, etc. Also, the bleed hole was cleaned out, and I made sure the rest of master cylinder was washed out well with solvent. Is there anything obvious I've missed? What makes brake fluid leak out the back of a front master cylinder? My first guess is to take it apart again and make sure the piston bore is squeaky clean. Thanks for your help

[82Shafty]

After you got everything squeaky clean, did you lube the piston/seal before installation?

I've seen new piston seals get whipped out in one swoop in a dry bore.

Brake fluid/silicone brake lube are good lubricants to use during a rebuild.

[DPage]

OK thanks, I think I used a little brake fluid for lube, but when I try again, I'll use a lot.

[SqDancerLynn1]

The bore could be pitted ? IF the piston was damaged, it will have to be replaced again. No amount of brake fluid will fix it. DID you use OEM parts

[almarconi]

I have the same problem with mine. Rebuilt with oem parts and there is a very small leak. The fluid runs into the brake lever switch area and a small amount runs down the lever. I bought another mc from an 850L and it had several small parts ( a plastic washer and a metal clip) that went over the little boot that goes over the mc plunger. My 750L did not have these parts. I don't know if the PO rebuilt the MC and removed these parts. They are not included in the OEM rebuild kit. Piston is in the bore> metal washer>circlip>rubber boot>plastic washer>metal clip. Not sure why this was used on 850 and not 750.

[chef1366]

Did you change the o-ring under the reservoir.

[SqDancerLynn1]

I have the same problem with mine. Rebuilt with oem parts and there is a very small leak. The fluid runs into the brake lever switch area and a small amount runs down the lever. I bought another mc from an 850L and it had several small parts ( a plastic washer and a metal clip) that went over the little boot that goes over the mc plunger. My 750L did not have these parts. I don't know if the PO rebuilt the MC and removed these parts. They are not included in the OEM rebuild kit. Piston is in the bore> metal washer>circlip>rubber boot>plastic washer>metal clip. Not sure why this was used on 850 and not 750.

That sounds like a good part to check, washer would put the piston just a tad more probably sealing

[almarconi]

Does anyone know if the rubber boot(it looks like a hat) that goes over the end of the mc piston is needed? Does it just keep dirt out of the mechanism or does it actually seal the piston so it doesn't leak.

[chef1366]

Dust seal only.

[DPage]

OK folks, I'm back in business thanks to everybody, especially almarconi. The key was the parts installation sequence, because I didn't know what to do with that white nylon washer. Piston>metal washer>circlip>boot>THEN the plastic washer slipped over the boot and pushed down into the bore. I got a new boot (the one shaped like a tiny Abe Lincoln hat) with the OEM rebuild kit, but not the nylon washer, and the original washer was in bad shape. The Suzuki dealer did not carry them, but I got a workable one from the hardware store. Start out with a 1/2"ID by 3/4"OD washer, then carefully sand down the outside diameter to .738", enough to fit firmly in the bore. Use an Exacto knife to open up the inside diameter to .596" - just enough to fit over Abe Lincoln's "hat". By the way, that "hat" boot has a metal metal doughnut inside it at the bottom, and I suspect it may have some sealing quality to it when you push it down into the bore. The nylon washer just keeps the "hat" boot in place. There was also a question earlier about whether the reservoir O-ring had been replaced - it had, but I failed to mention that, and it is something that could have led to leakage, which was my original question. This is a great forum!

[almarconi]

Did you install the wire clip on top of the plastic washer? I wrapped a bungee cord around my brake lever a couple of days ago to see if I could get it to leak. Seems to have a very small weap where the bottom of the reservoir meet the mc body. The lever appeared dry. I'm probably going to pull the mc this spring just to be on the safe side.