Topic: Help - Master Cylinder Cap Leaking 78 750K8

[cjackel]

I removed and cleaned front caliper, changed brake fluid, bled brakes and adjusted caliper.

When riding, the master cylinder leaks fluid at the cap. This was happening prior to the above work.
I thought it was due to the brake pad/piston being stuck causing the fluid to overheat
and boil in the master cylinder.

The fluid level is right at the line. I removed the rubber diaphragm and plastic washer from the cap and
reinstalled. I still have the leak. It only leaks when bike is running and seems to occur at speeds
above 20mph.

The brake disc seems to be warm but not hot after riding.
Any ideas what's wrong?

[SteveD CB500F]

Has the rubber diaphagm perished?

I've seen this on an old Honda 250 which leaked from the cap.

[cben750f0]

had a similar prob with the F1... it was a perished diaphram... couldnt really find it off the bike, but when the lid was screwed down, caused the crackes to open up under the cap... leaking fluid... see if it happens with a new diaphram... cheers

[cjackel]

I replaced the rubber diaphragm with a new part from
the honda dealer.

Master cylinder still leaks at the cap, not as much, but still
drips when on freeway at 50mph or greater. The brake fluid
runs up onto the cap or drips down the side of
the master cylinder.

Any ideas?

I removed and cleaned front caliper, changed brake fluid, bled brakes and adjusted caliper.

When riding, the master cylinder leaks fluid at the cap. This was happening prior to the above work.
I thought it was due to the brake pad/piston being stuck causing the fluid to overheat
and boil in the master cylinder.

The fluid level is right at the line. I removed the rubber diaphragm and plastic washer from the cap and
reinstalled. I still have the leak. It only leaks when bike is running and seems to occur at speeds
above 20mph.

The brake disc seems to be warm but not hot after riding.
Any ideas what's wrong?

[n9viw]

Check for nicks on the lip or threads of the cap. If there's a flaw in the sealing surfaces, the brake fluid will seep out. Run your finger along the gasket surface of the MC reservoir, and if you find one, use some 600 grit wet-dry sandpaper and smooth it out.
If you don't have any snags, check the cap for cracks.

[frostypuck]

Sure the plastic washer is in the correct place? I think it goes: rubber, plastic, cap. The plastic washer holds the rubber diaphragm down tight to the top of the m/c I bought a bike a while ago, that had the rubber pushed thru the opening in the plastic. Go figure. It leaked like crazy.
Chris in Boston

[eldar]

I have to ask where EXACTLY it leaks from. I did not tighten my banjo bolt(why is it called that?) enough and it leaked between the washers. Otherwise, are you tightening the cap enough? Sounds simple and dumb but the cap should be hand tight not just finger tight.

[cjackel]

The leak is from point where the threads of the master cylinder and cap meet. In other words, it LOOKS
like a leak due to not being tight, but I had it pretty tight and it still leaked.

I looked at the sealing edge of the master cylinder and it has some old, dried up fluid or "gunk" so it might
be the source of improper sealing.

The order of the parts as its installed now are rubber diaphrapm on master cylinder, plastic piece in cap and then cap.

No cracks appear to be in the cap, but I guess its possible. I'll try your suggestions with the wet sanding and
try again. Thanks. I'll keep you posted.

[cjackel]

After replacing diaphragm, sanding master cylinder reservoir edge, removing caliper piston, cleaning square oring,
replacing brake pads, adjusting caliper and replacing brake fluid, the master cylinder still leaked at cap.

I even tried tightening the cap by hand until I could turn it no more. It still leaked.

I rode around for 10 miles and stopped into the local Honda dealer. I bought another rubber diaphram,
but this time I also bought a new plastic ring (goes inside the cap, on top of the diaphragm.

I rode home and took the cap off. The original plastic ring and new ring are different. The new part has
an additional lip on the inside diaphragm. It appears to put more pressure over a larger area of the diaphragm,
and in turn gives a better seal.

I replaced everything, tightened down the cap and test rode. No leaks!!!

Thanks for everyone's help and suggestions.

[cjackel]

200 miles after replacing the master cylinder cap diaphragm and plastic washer.
I have zero brake fluid leakage.

Thanks for the help guys!

[bransonmowoodman]

Im having the same problem on my 78 750k honda, guess i will replace the 2 parts, if i can find them at the honda dealer. leaks a few drops after you start riding, and just painted the tank. Rick