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MG MGB Technical - bendix brake servo 77 mgb 1.8

Hi out there ,when braking two things occur
1)There is a whooshing/air sound from the servo...
2)The engine wants to stall/falls in revs to point of stalling.
Ant suggestions....? Moss Denamark...
I am a very new mgb owner (thus no experience with anything mechanical)- by the way a great forum...this must be the spirit of mgb owenership I got told about...
JRB Moss

Sounds like the main diaphragm or air-valve diaphragm is punctured, or the air valve is stuck. Either can let air into the inlet manifold when the brakes are applied which can have both effects.

Without braking manifold vacuum is applied to both sides of the main diaphragm so has no effect. When braking the passage from the front to the rear of the diaphragm is closed by one part of the air valve, and another part of the valve opens the rear of the main diaphragm to atmosphere. Atmospheric pressure on the rear, and manifold vacuum on the front, causes the diaphragm push-rod to push directly on the servo piston. adding to foot pressure. You might like to have a look at

As a diagnostic/temporary measure you can disconnect the hose from the servo and seal the hose, which should prove the point. But note that whilst the remote servo only gives light assistance and you will barely notice its removal, if you have the integral servo you will almost certainly find the brakes much heavier than before, so take care.
PaulH Solihull

Hi PaulH, thanks for the advice - also on the other issue with shk, shk noise - seemed to have two threads by accident.
I'll have to read the manaul before looking at the brakes - very inexperienced with this stuff. Thanks for the link - I'll read it at least thrice!
JRB Moss

PaulH, read your link on servo - thanks - it refers to another type of servo - mine is the bendix type on the tandem master cylinder with PDWA. I get the bit with the membrane not creating enough pressure - but the part of the servo with what I presume is the membrane is sealed (that is I cannot see any screws to undo)Unless of course I have to take it out first then look. Moss
JRB Moss

Neither of my cars have that type, they were only used in the UK from 77 on and I haven't worked on one yet, I'm sure someone here has. There are various types, either a clamping ring of various types, a set of screws (although perhaps only on the MGC) or you may have to use a special tool to twist the two halves of the vacuum chamber apart, to reveal the main diaphragm. You will have to remove the master cylinder from the servo chamber at the very least, and that may you require to remove both from the pedal box first.
PaulH Solihull

Haynes gives a step by step on how to take the servo out - how I seperate the two halves I'll have to find out when its on the bench. Thanks again for your response.

You can split the two halves easily by removing the master cylinder, attaching a long drilled bar or length of dexion to the studs on the front of the chamber and GENTLY turning the unit. You will see that the two halves engage with an indexed keyway which pushes the halves together. you will need to jiggle the front in order for it to dis-engage, and remove it carefully in case the contents go everywhere under spring pressure. Possibly not orthodox, but it was almost impossible to split the unit off the car as we couldn't grip the round case securely enough. Once it's in two parts you can either work on the base on the car, or undo the four studs within the pedal box to remove it.
Pat Gregory

Thanks Pat. Now my total lack of knowledge on things workshop stuff comes to the fore - what's a dexion?

Strip steel bent into a right angle along its length, punched with various holes and slots. Used to construct shelving.
PaulH Solihull

This thread was discussed between 15/09/2010 and 19/09/2010

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