Morris Minor Owners Club

could i block the breather hole to stop leacking, if so what would hapen long term wise with the break fluid.

I'm not sure where you got you master cylinder from but I don't have a breather in mine - maybe that's your problem...?

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i do on the new top as well but should not leak from it only a pin hole

(a) DON'T block the vent hole in the cap.
(b) This is a classic sign of master cylinder failing when you get the "fountain effect"

Sorry to be negative but take it as a warning sign that brake failure could be imminent and immediate investigation / rectification is needed without delay / further driving.

Paul H.

Can you expand further on your statements Paul? The picture I posted above is an original master cylinder with no breather in the cap. Every car I've worked on will expel fluid with the cap off and I've not suffered from any brake failure? I'm not fully understanding what you mean?

Every standard Minor Master Cylinder has a pin-hole breather in the cap. If it's not present then either the cap has been deliberately modified for use with a remote reservoir (if the reservoir is higher than the M/c then fluid will leak out of the breather) or, more likely, the hole has become blocked with crud - after all it leaves in a pretty grimy environment!

Leyton, could you describe the problem in more detail? Are you seeing a small fountain when the cap is off and your depress the peddle. or is there fluid leaking out with the cap on? If the former, then perfectly normal, if the latter then it could be that the cylinder is overfilled (the fluid should be 1/4 - 1/2 inch below the neck of the filler) and/or the sealing washer is damaged or missing from the cap.

Silly question: Why have we got two threads on the same subject? Could a Mod please combine them. Thanks.

My cap has a vent hole through it (about 1/16in)
This is the hole I pressurise the system through (about 8psi) when I pressure bleed the system - works a treat!

PSL184 wrote:Can you expand further on your statements Paul? The picture I posted above is an original master cylinder with no breather in the cap. Every car I've worked on will expel fluid with the cap off and I've not suffered from any brake failure? I'm not fully understanding what you mean?

You press the pedal and you want the fluid to go down the pipework to the wheel cylinders.
If it comes backwards into the reservoir on the downward stroke then there is something wrong such as a seal the wrong way round or even one failed if there are several on the piston. Worn / damaged bore can also be a cause.
A little "displacement" like you describeon the return is normal but when it gets to the point where someone needs to consider blocking off the vent in the cap to stop it leaking out then there is something that needs investigation.
After all I think we'll all agree fluid shouldn't be comming out of the vent in normal operation so better to advice someone to investigate

Paul H.

Yes, agree - it shouldn't be coming out of the vent (even if I'm not entirely sure there is a vent on all the caps, but I'll bow down to that one). I'll settle on a "little displacement" as being the norm that I am used to seeing I think the original statement made (maybe on the other of the 2 threads) was that the MC was new - hence my reason for asking where it was purchased. Should also ask if it is a genuine one or a Chinese copy ??

The little fountain is absolutely NOT a sign of anything wrong with the master cylinder!! They all do it - it's not a problem. You must have the vent - don't block it. You just have too much fluid in the master cylinder. If the fountain is excessive - it may need the pushrod adjusting - but assuming it was working ok before you worked on the brakes - it will still be ok.