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MG MGB Technical - Pressure Failure Switch

With regards to 75 B brake system.
I have what I call a proportioning valve which is mounted on the drivers engine compartment fender. The brake lines from the master cylinder enter it and from there the brake fluid is sent to the front and rear systems.
I have rebuilt the master cylinder and during the rebuild removed the Pressure Failure Switch (the plastic thing which screws into the proportioning valve). Now brake fluid is leaking through the plastic switch, not around the threads but right through the switch itself. I was wondering if this is because the plunger of the switch is not sitting on the ball in the valve (the system hasnt been equalized yet)
Bruce

No.

The thing you call a "proportioning valve" is really the hydraulic mechanism that physically activates the
electrical Brake Failure Switch. It does very little in the way of regulating or proportioning brake
hydraulic pressure.

The body is shaped like an "H" and when there's a sudden drop of hydraulic pressure in either the
front or rear brake system - a "shuttle piston" slides across inside the center of the "H" and pushes
up a plunger to activate the electrical switch.

There is a tiny rubber "O" ring on either end of the shuttle piston which prevent brake fluid from
finding it's way up into the switch itself.

It sounds like those "O" rings are bad and need to be renewed.

Fortunately, the rebuild kit for this is inexpensive and the procedure is straight-forward and not too
difficult (...at least it's far, far easier than rebuilding the master cylinder).

Robert Bentley's "The Complete MGB" gives details on how to do this.
Daniel Wong

Bruce,

Daniel is right. I just finished that last month on my 68 B. Very straight forward and simple. The hardest part was keeping the valve centered when I filled the new brake system. I filled the resivor and let gravity do its job for about 2 hours then blead the system per the Haynes manuel.
Cris DeYoung

Thanks guys/Daniel .I have ordered a new kit. Apparently I have to take the ‘H’ out to rebuild the switch. As I just replaced the master cylinder and all new brake lines, it took a while to bleed. Guess I will have to do it all over again.
Won’t be as bad, at least there will be some fluid in the lines this time.

Thanks

Bruce

Bruce

These valves have a critical function besides operating a warning light.
With master cylinders that that do not have tandem pressure chambers, and therefore only have one pressure line coming from them, when a brake hose fails the shuttle piston moves across and cuts off the fluid from the affected brake line pair.
The other two brakes still work and the new shuttle piston position prevents all the remaining master cylinder fluid being pumped out.
The warning light is also activated to tell you that you are on two wheel braking only.

Mick
M F Anderson

Mick, 68-74 1/2US cars had a dual master cylinder with the brake failure valve. The valve switch will turn on the dash brake failure light if one circuit fails but the shuttle does not shut off fluid flow to the leaking circuit. The dual reservior on the master cylinder prevents fluid from leaking out of the good circuit. Maybe the valves used in Australian cars are different. FWIW, Clifton


Clifton Gordon

This thread was discussed between 16/06/2003 and 17/06/2003

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