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MG MGB GT V8 Factory Originals Technical - Brake Booster rebuild?

My brake fluid was 'disappearing' and I went right through the system and couldnt find a fault. I rebuilt the master cylinder and finally found today that it is leaking through the booster and into the inlet manifold and putting it out as smoke. I have tried stripping the remote booster and got to a certain point but cant figure out how to get the diaphram mechanism out, I assume there is some seals or something behind it possible to rebuild, If so any hints and does it require special tools? Replacement boosters are expensive and even the kit isnt cheap, I dont want to buy a kit if I cant get it in.

Your advice would be appreciated

frank swinton

I presume you mean the main servo diaphragm since the air valve diaphragm looks fairly straight-forward - just remove several screws. For the main diaphragm, and hence the seals on the servo piston, you have to turn the end cover wrt the main body of the servo anti-clockwise until its fingers come out from underneath lugs on the main body and the cover can be lifted off. The book shows a large special tool for this, being bolted on to the cover and using a socket bar to turn it. The bolts simply hold the tool to the cover, they do not take the turning forces, recesses on the tool engage with ribs on the cover to the turning. This implies that some force is needed even on a new one. Have you tried driving without the servo? It gives surprisingly little effort, after a trial prod of the brakes on a friends V8 with a disabled servo as it kept locking on I just didn't really notice its loss. It should be an MOT failure though, if fitted but not working, as opposed to completely removed.
Paul Hunt

if you are not worried about stock items then perhaps you could just replace the servo with one from a modern car.
Cecil Kimber


on the remote servo there is a band that runs around the circumference and allows the 2 halves to separate and the large diaphram is sandwiched in there. I am assuming the serviceable parts are behind the plunger which is inaccessible due to the diaphram being in place. How do I get the diapham off?

I will put in a temp pipe to bypass the servo tomorrow so i can move the car and try it out.

What book are you refering to as there wasnt much in the Haynes or restoration books about the servo...

frank swinton

Frank - I was going by the Leyland Workshop Manual (e.g.
which shows a Lockheed Type 6 remote servo the rear cover of which has to be unscrewed from the main body. Haynes just says what my 3-year old granddaughter would say - "It's a bit difficult". I didn't go and look at either of mine I must admit, but now you mention the clamp I seem to recall that is what mine have but the manuals I have don't cover that one. However the Parts Catalogue mentions two types, only showing the later one which has the clamping band. In this the diaphragm is cup-shaped with quite deep sides which will lie against the inside surface of the main 'can' of the servo. I could quite imagine this is just a push-fit and despite being rubber has rusted to the body over time, so careful levering between the two may free it. I'll email you a scan later on today from the Parts Catalogue in case that helps, and also from the Workshop Manual as the 'innards' should be much the same I would have thought.
Paul Hunt

Frank, from memory, the diaphram has a bayonet type catch.You press the diaphram against the spring & twist it left or right ( can't remember which).Then it should come away. You can get a repair kit from any MG parts supplier. It might even come with a detailed diagram. Barrie E
Barrie Egerton

Thanks Paul and Barrie,

I will try pushing and twisting and see if I can get the diaphram off. I temporarily bypassed the servo and you were right, the brakes are better......very responsive and a positive feel. Also soooooo much easier to bleed.

frank swinton

If you are willing to get help from Truimph go to for an extensive article on the repair of Brake Servos. I am willing to bet Triumph and MG bought from the same supplier. I own a TR6 and MGBGTV8.
Richard Porter

This thread was discussed between 11/01/2004 and 14/01/2004

MG MGB GT V8 Factory Originals Technical index

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