Topic: Changed brake hoses, master cylinder cap leak (on relatively new MC)

[Chachi]

I put on a set of Slingshot SS hoses today. Everything seemed to go OK except I have a leak on my master cylinder cap. It was not there before the hose swap. Also, the MC is less than two years old.

It's this one - http://www.cb750cafe.com/products-page/brake-system-parts-hot-rod-parts/cafe-racer-brake-master-cylinder/

I didn't get it from Carpy, but it's the same MC.

Anyway, I've checked out the diaphragm and plastic ring that is on the underside of the cap and everything looks OK. Other than possibly trying a new diaphragm and/or plastic ring, is there anything else to try?

Only other details to mention... I did have to take the MC off to remove the banjo bolt at the MC. It was on there real good. With the MC 'empty' I had it resting upside down with the cap on to work on the banjo with a socket. Until this hose swap (and MC removal from handlebar) the MC has never had a leak. I checked fluid level - initially it was a hair too high so I removed excess with a straw so it is right below the fill line and the MC sits near-perfect level.

[Harsh]

It may sound odd, but I have seen the rubber diaphrams actually absorb and eventually pass fluid through them.  I have an issue with it on my Rizoma master cylinder on my Triumph.  What I have to do is occasionally take it out and clean it with some brake fluid cleaner.

Is your location correct, Norfolk VA?  If so I am local and wouldn't mind meeting with another SOHC4 memeber.

[Chachi]

I guess a quick spray of brake parts cleaner won't damage the rubber? When it first leaked I did have fluid slightly above the fill line, however it still leaked when I removed some fluid to get it below the fill line. I also tightened down the cap as tight as I could thinking that was it, but I've read some posts that said over-tightening the cap can make the diaphragm not seal correctly. I'll re-fit my cap today and try it again, if it leaks I'll try cleaning my diaphragm and I also have a spare diaphragm from a salvaged MC to try.

Could having the MC upside down (cap on bottom) with some residual fluid in the reservoir have caused this? I know some fluid got on the cap side of the diaphragm. Should that side be 100% fluid free?

This was my first hose swap and brake bleed. I anticipated an issue with bleeding or a leak at a banjo, but not my cap.

[Chachi]

I cleaned the underside of the cap as well as the diaphragm and plastic ring - wiped them out, hit them with some Brakleen. That seems to have done the trick as I was able to do a 10 mile loop with no leak. Yesterday I couldn't even get out of my neighborhood. Maybe the Brakleen swelled the diaphragm, maybe I just needed to get the residual fluid off of the backside of the diaphragm - or maybe my bike just wanted to keep me in check.

My loop was to my Honda shop and back to pick up a replacement diaphragm (they had one in stock). I passed on it since I didn't have leak. The told me that they replace MC diaphragm's anytime they do a fluid change - liability. They've been burned by a tech actually leaving a cap off and a customer riding away.

[Harsh]

Glad to hear that it sounds like you may have been able to get it fixed.  Yes the side above the rubber diaphragm should be 100% fluid free.  You may have just had some brake fluid in the grooves in the cap and it was squeezing out when tightened.