Topic: master cylinder leaking at cap

[carl550k]

my 76 550 has had clubman bars on it since last year. this year after rebuilding the used one from e-bay i now have a slight leak from the cap. i thought it may be the rubber diafram? under the cap, but since i have 2 now i have tried both with the same results. i even switched the caps with no luck. could i have overtightened the master cylinder cap and destroyed them? since the master cylinder sits at an odd angle because of the bars i run just enough fliud to cover the holes at the bottm of the resviour. today after driving over some bumpy road i noticed some fluid trickling down the side, not much but at speed this blows onto my tank thus slowly destroying my paint. any ideas? thanks carl

[750goes]

wrap a plastic bag or some plastic wrap around it....

[Jinxracing]

wrap a plastic bag or some plastic wrap around it....

Golly, that's useful. 

I don't know the answer, but I'm interested to know what others think. I've been concerned about the angle that my MC sits at when mounted on my clubmans. I haven't gotten to the point of completing the brake system yet, but I've been wondering. Do you lean the bike over far to the left to get the MC full? 

[brandon]

It could be that you simply over torqued the cap down. As an example, my master cylinder is damaged somewhere in the mechanism yielding my front brake useless. It sits in a bag with fluid still in the whole system. The cap does not leak at all. Still full of fluid.

Anyways, short of replacing the whole mechanism, what comes to mind for me is working out some kind of bushing that can be used in the cap. NAPA sells various sizes of rubber O-rings that may be beneficial in this application.

What I'm thinking is taking an O-ring that is the same diameter of the master cylinder cup and place it so it sits on the cup, then carefully slide the cap over it and tighten slowly. If the O-ring doesn't seal it (may want to slip and roll over part of the edge) , try making something scimilar out of a softer more malible rubber. Essentially you're just making a gasket.

[cadler]

do you have the rubber piece that goes on with the cap?

[Steve F]

I've seen the rubber diaphragm get a small crack or even a tear that would allow the fluid to escape, but then you would see evidence of this with fluid building up on the top of the diaphragm.  If I remember correctly, there's a small "notch" molded in the top side of the diaphragm lip that allows the air to get in the top side of the diaphragm under the cap to allow the diaphragm to extend down into the reservoir when as the fluid level drops due to pad wear or maybe leakage.  If the cap is too tight, you may be squishing the lip of the diaphragm so much that the "notch" becomes closed off, and won't allow the diaphragm to flex, and may be part of the problem, causing the fluid to get pumped out under the lip if you should get the brakes hot or if you replaced the pads recently.  Just so you know, there's supposed to be a plastic ring that goes between the metal cap and the diaphragm.  For my '76 750F, the part no. is 45521-300-010.  This ring is supposed to maintain the "notch" clearance so you don't have problems like I described.

[carl550k]

 steve f , i have found fluid on top of the rubber diafram, but only on one of the 2 i have tried. the plastic o-ring is in there. again i have tried both with no luck.i have even tried to line up the 'notch' under the cap at the highest point thinking this could be the culprit, no luck. and yes, if you lean the bike over to the right fluid will begin to weep out from under the lip of the cap.