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MG MGB Technical - Brake Master Cylinder- No Pressure Failure Switch!

The PO strikes again!! I have a 76B, and I was getting ready to bleed the brakes - they are a little spongy, but the pedal doesn't go to the floor, so I figured I'd try a bleed before I started suspecting the master cylinder.

I started checking for the pressure failure switch, so I could back it out 3 1/2 turns as the manual directs, and to my surprise, all I have is a bolt in the bottom of the master cylinder! I found the connector that is suppose to plug into the switch in the engine compartment, but it's not plugged into anything.

My question: Can I continue to drive this way? I've put 2000 miles on the car since I got it, including stopping on a dime for a deer one night. Can I bleed the brakes, and just not worry about the pressure failure switch?

Boy, am I tired of fixing things because some guy didn't take the time to do it right!!

John English

Hi John.

The switch is a safety device, so it only provides a little reassurance UNTIL you get a leak in the hydraulic system, at which time it will let you know about the leak. It effectively does this by detecting a pressure difference between one brake hydraulic circuit and the other.

I don't think it will affect the actual operation of the brakes, but I am not sure about this, could it allow the master cylinder pistons to move too far if the bolt isn't long enough, or if the bolt is too long could it foul on something ?. My advice is to replace it immediately.

I actually like putting PO related probs right, it might be a pain at the time, but is rewarding when it is done !.




I've usually seen the brake pressure warning switch removed for one of two reasons:
1. The switch broke and the DPO pulled out the remnants and installed a bolt in it's place.
2. The seals around the shuttle piston (sorry, don't know what else to call it!) have failed and brake fluid was leaking out of the pressure warning switch.
Again a bolt was installed to "fix" the problem.

My guess would be #2 since the warning switch is located under the MC and is difficult to break off accidentally. If it were mine, I'd pull the MC and rebuild it. If for nothing else, just the piece of mind.

Good luck,
Tom Sotomayor

Don and Tom -

Thank you for your insights. I was planning on doing a rebuild eventually, so I guess I'll have to go ahead. I can't believe the price of the plastic pressure failure switch, though - $35! The rebuild kit is only $18!

John English

This thread was discussed between 12/08/2005 and 13/08/2005

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