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MG MGB Technical - brake servo

1974 MGB been standing 4 years or so. Drove to MOT ok but on brake test locked up solid. Disconnect servo brakes released and drove home with fail ticket.
It seems likely to be the servo and probably the air valve. I have taken the top and rubber diaphram off but how far can I strip to get access to the air valve and if I can could I clean from outside (without taking the unit to bits) and should I try and use silicon oil/grease to free up. Any advice would be appreciated as I cant see a thread on the subject
K G Penna

It could be the air valve piston that is sticking. To remove this you will need to replace the diaphragm and the top and put the screws in a couple of threads only. Then depress the brakepedal by hand so as to partly eject the piston then remove the top and diaphragm again and remove the piston. Polish it with a very fine wet or dry paper and clean it thoroughly removing all traces of abrasive. Now put a dod of silicon grease in the port and refit the piston. You won't need to bleed the brakes and hopefully this will have solved the problem. I assume that the 74 car has a remote servo and if you are using silicon fluid this tends to make the servo stick and leave the brakes on. The best thing you can do here is to replace with Dot 4.0 brake fluid.
Iain MacKintosh

Thanks Iain, I will give your suggestion a try tonight but as far as I remember the hole where the plastic plunger from the diaphragm goes down (presumably)to the piston is very small. Is the piston small enough to come out of that?
K G Penna

The piston is very small diameter and comes out of the body of the main casting. You will see it when you depress the brake pedal.
Iain MacKintosh

Thanks again worked a treat brakes back to normal and MOT pass
K G Penna

Glad we have some satisfied customers. Good luck
Iain MacKintosh

This thread was discussed between 02/08/2011 and 04/08/2011

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