5 replies to this topic

[Sask_Mini]

Hey Guys, 

 

So I finally had time to take the mini out of its winter hibernation and I was checking over a few things.

 

I noticed that there was a small amount of brake fluid at the base of the master cylinder.  Its the tandem style master cylinder with the brake fail switch on the body of the master cylinder.  I wiped the fluid away and pumped the brakes a few time and it looks like the fluid is leaking through/around the brake failure switch.  I did some research before posting here and it sounds like under normal conditions there shouldn't be any brake fluid in contact with the brake failure switch.  Does this indicate that one of the seals in the master cylinder has failed and is allowing brake fluid in behind the brake failure switch?  

 

The brake failure switch isn't hooked up to any wiring currently, I have only owned the car for a year, so I'm not sure why this switch isn't hooked up.  It actually looks like the end of the switch that connects to the wiring was broken off at some point.   

 

If someone could verify that my diagnosis of a faulty seal in the master cylinder is correct or if someone could point me in the proper direction that would be great! 

 

Thanks,

[Spider]

Short answer is there is an issue and yes, there are seals which the fluid is leaking past.

 

There's a Pressure Balance Shuttle in the Master Cylinder that activates the Switch. The Switch itself is fitted in to an area that is dry of fluid. There are O Ring Seals on the Shuttle to make it work and to seal the fluid in.

 

It's unusual for the O Rings themselves to fail and leak, it's highly likely the the bore in the Master Cylinder in which the Shuttle is fitted to has rusted. It is quite small in diameter (about 1/4" off hand). It's been my experience that once any Brake Hydraulic Cylinder leaks from rust, while the surface rust can be removed and the Cylinder re-built with new seals, it's only a short space of time before the rust returns. You'll also likely find that if this has rusted to the point of leaking, then the rest of the Cylinder probably isn't in good shape either.

 

I'd suggest a new Master Cylinder.

[tiger99]

I totally agree. The bore will be worn and corroded and the new seals will have a very short life.

I would suggest changing to the more readily available and cheaper later type master cylinder with the float switch sensing fluid level in the reservoir. That removes dangerous failure modes like the one you have from the pressurised part of the system and is the approach used on modern cars. You do neet a lamp test switch, but you probably have that already. Unless of course it is a rare car and originality matters.

[sledgehammer]

+1 on the above posts

 

has anyone fitted a buzzer to the low fluid light ? , on a bright day , the light on the switch is hard to see when driving

 

Edit - although noise in the cabin will probably down it out anyway

Edited by sledgehammer, 07 May 2017 - 09:49 AM.

[Spider]

+1 on the above posts

 

has anyone fitted a buzzer to the low fluid light ? , on a bright day , the light on the switch is hard to see when driving

 

Edit - although noise in the cabin will probably down it out anyway

 

No Buzzer on the Brake Fail / Low Fluid Level indicator, but I have one on the Low Oil Pressure - and yes, I can hear it above everything!

[Sask_Mini]

Thanks guys!  I already have a bunch of other parts I need to order so I will add a new master cylinder to the list.  I'll probably upgrade to the later type master cylinder switch as tiger99 has suggested.  I think mini spares has a conversion kit that makes the upgrade pretty easy.