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MG MGB Technical - Brake pressure failure switch

First off, I am happy to announce that I just purchased a second MGB 2 weeks ago.

It is a 1972 MGB-GT. There are several things needing attention.

First and foremost it is leaking brake fluid from the brake pressure switch at the presure valve assembly. It is a small drip and the brakes seem to work fine and the peddle is firm so I don't think any air is getting into the system. I've been told by my parts guy that the problem is the seals in the pressure assembly not the switch itself.

My question is, can I just remove the switch and put a plug there in it's place? I don't care if I have a warning light working. What is the purpose of the brake pressure assembly? As I have said the brakes appear to be working just fine. (They even feel more firm then the ones on my 1979). I'm just loosing a little fluid that requires topping up of the brake master cylinder.

Thanks for your thoughts.

John Fraioli
1979 Inca Yellow MGB
1972 Blue MGB-GT
John F

John. My 68 GT, using the same braking system, has had a bolt to plug the hole for the warning system switch since I purchased it. Since the wires going to the switch carry current whenever the ignition switch is turned on, they need to be insulated and tied up out of the way if the warning switch is removed from the circuit.

In over 40 years of driving, I do not remember a "brake warning light" ever giving me a warning of impending brake loss. I have had the lights come on with no problem found several times. I have, twice, had rebuilt wheel cylinders fail with the loss being noted in the feel of the brake pedal before the light came on. Thus, I do not worry about the working condition of the light. Others may.

Les Bengtson

Thanks Les,

That was the way my toughts were going also. Just wanted to make sure there wasn't some other function to the pressure assembly before I removed the switch and plugged the opening. After reading about the assembly in my Haynes manual, it appears that the only function of the pressure assembly is to activate the switch via a sliding plunger which moves one way or the other depending on a difference of pressure between the dual braking systems. I'm thinking that the worst case scenario of not replacing the seals within the assembly would be that the system would be pressurized more as a single system rather then a dual system. Does this sound correct.

Can you tell me the thread size of the bolt I would need to plug the opening?

John F

John. The thread size on the example in my spare parts bin (which has the switch present) is 3/8"-24 UNF. Length of the threaded portion is .260", or just a hair under 1/4" (.250"). If you can find a 3/8"-24 bolt in the quarter inch length, it should work fine.

As to the theory behind how this system works, I do not know. Have not taken this system apart. You are correct for the later style master cylinders which I do have experience with. In the later style master cylinders, the ones having the switch integral with the MC, the fluid flows through the left hand section which has the rod and switch in it. If one side has a pressure loss, the rod is forced towards the side having less pressure and the switch comes on when locking the rod in place. This seems to be the same mechanism that is used with the brass five way connector in the 68-76? cars. But, I cannot be definitive until I have a chance to take one apart and examine it.

Les Bengtson

John, if you don't care about the light just plug it and forget it. I did put new seals in my 69 when I went to silicone fluid 10 years ago. The valve is basically a rod with needle taperred ends and a "V" grove in the center with each side sealed from the center. If one side of the dual system springs a gusher leak then the rod will slide to the low pressure side and push the the switch plunger up thus making contact and completeing the electrical circuit. The leak occurs when the fluid gets by the seals and works its way out the switch. Putting a bolt in place of the switch will have no affect on the braking perfomance. There is another type of valve (I've disassembled both) that looks more like a bar bell on the ends and uses a different type of seal but the principles are the same.


Here are some photos of the brake failure system. Both types shuttle are shown.
When removing the switch and plugging the hole I use a copper washer on the bolt to prevent leaks. Bad seals will allow some fluid to go both ways. That could cause problems if you have a leaking cylinder/caliper or other brake component. If the seal leak is a big leak it could possibly allow both cylinder reservoir compartments to drain and you would have no brakes. The leak is usually so small it's not a big concern.

Clifton Gordon

I put a plug in mine last year and there hasn't been a problem.
1977 MGB
randy olson

This thread was discussed between 06/04/2007 and 09/04/2007

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