Master cylinder leak query

[Ginja]

Hi all,

I upgraded to discs on my 69 Coronet using a kit with wilwood calipers, a bendix booster and master and oe style master cylinder from Cass. Kit works great, but I've an odd leakage problem from the master cylinder cap - wondered if any of you had experienced it?

If I start with a clean system - dry seals and cap lid on the reservoir I can drive the car for a day with no issues. The next time I use the car it will leak from the cap at some point (not sure at what point - driving or cooling down). On removing the cap after leaking the diaphragm in the cap washer will be 'down' into the fluid. My theory is, first run fluid gets hot, expands, air is forced out (there is a tiny channel in the inner cap lid to facilitate this). Fluid cools, and contracts sucking down the diaphragm. Next use of the car this acts as a pathway to the lid for the fluid which is then pushed out when the fluid expands. My question is, what am I doing wrong - is the fluid too high (about half an inch from the top - enough to connect with the diaphrgm when 'down') or may I still have air in the fluid somewhere? There is no spongyness to the pedal - well, not that I can notice. Car does have hooker headers and a choppy cam so gets hot under the hood, but I don't beat on it.

Any ideas folks? Thanks!

Mark

 

[steve340]

That diaphragm is designed to pop down - I read somewhere it is too take up the air gap above the fluid to minimise contamination.
Make sure the lid etc is actually flat compressing the gasket and the clip is tight.
Run the fluid a little lower and try that. The slightly lower level will not worry the brakes at all.

 

[pnora]

What mater is it? One with a lid or one with two caps?

 

[Ron H]

Not sure what your MC looks like; but possibility of the cap not fully seating or seated. Check it on a level surface and also watch the action of the lid when locking it down...not springing up anywhere around the seal. Assume you bench bled the MC...when you nail the brakes you don't get any action of the pedal surging or pushing back? This can be an indication of trapped air in the MC. Ok, I know these are just a couple of stabs; but tossing in my two pennies..

 

[Billccm]

I have the Dr Diff conversion kit with 15/16 master cylinder. I too have some seepage around the front cap. Fluid level is what I'd describe as 3/4 full.
I took a silicone gasket and placed it under the stock rubber cap seal. This makes putting the lid on very tight. Still have the seepage. I'm just in the habit now of wiping the master every other outing.
I think over on FABO several people are complaining about this issue with no resolutions.

 

[pnora]

I have the Dr Diff conversion kit with 15/16 master cylinder. I too have some seepage around the front cap. Fluid level is what I'd describe as 3/4 full.
I took a silicone gasket and placed it under the stock rubber cap seal. This makes putting the lid on very tight. Still have the seepage. I'm just in the habit now of wiping the master every other outing.
I think over on FABO several people are complaining about this issue with no resolutions.

So you have the plastic twin cap master cylinder?

 

[Billccm]

So you have the plastic twin cap master cylinder?

Yes.

 

[pnora]

Yes.

Every one of the plastic capped reservoir's that I have seen leak replacing the rubber seal/diaphragm fixed the problem.

 

[Ginja]

Thanks all, this is exactly what I have - the twin capped plastic tanked reservoir with exactly the same symptoms. The lid seems to seal but when I originally fitted it I was surprised at how easy it was to fit the cap - not really a 'compression fit', but the seal bears witness marks all the way round the edge. Where might I acquire some replacement seals?

I wonder about just buying/fitting a wilwood MC and moving onto the next problem...