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MG MGB Technical - Servo non-return valve

Is it possible that the non-return valve in the inlet manifold controlling the vacuum to the servo could be stuck open? if so is there an easy way of checking this.
Can it be replaced easily should it be faulty?

This may help to solve my problem as my servo is out of action at the moment.BGB GT model 1978.
si robathan

The one way, or non return, valve should be located on the servo itself . By disconnecting the vacuum hose and applying suction, you should be able to discern if the servo is holding vacuum or if the valve is faulty. RAY
rjm RAY

The non-return valve on the manifold is there to prevent fuel and vapour getting to the servo. It is also better to "drop" the pipe below the manifold on it's route to the servo. I made a small trap (u Bend) out of copper pipe to put in the vacuum pipe as well, belt and braces!!!!
Allan Reeling

As Allan says the non-return valve should be part of the manifold port, not the servo. But it's there to keep a vacuum in the servo if the engine should stop while you are still moving, to give you two or three full normal operations of the brakes to bring the car to a halt safely. Note that whilst the MGB remote servo only gives light assistance, I believe the later integral servo gives significant assistance. Note also that if the momentum of the car is still spinning the engine i.e. you haven't knocked it out of gear or dipped the clutch, and you still have pistons going up and down and valves opening and closing, then you will still have manifold vacuum even though the engine isn't firing.

You can check the valve very simply by pressing the brake pedal several times after the engine has been switched off and stopped. For several operations of the brake pedal there should be a hissing and wheezing from the servo, lessening with each pump, and the pedal will start to get higher and harder as the wheezing lessens.

You can also check the servo itself by pumping till the wheezing stops, holding the pedal hard down, and restarting the engine, and the pedal should sink a bit.

If there is no wheezing, and if the pedal feels exactly the same with the engine running as with it stopped, then it's possible the servo isn't functioning at all, although that should give a very heavy pedal on your car and that servo. Depending on what has gone wrong (if anything) with the servo it may also be causing a significant vacuum leak into the manifold which will be affecting how the engine runs. You can check that by disconnecting the servo hose from the inlet manifold port *and firmly sealing that port off* and see if it makes any difference to the usual idle speed.
PaulH Solihull

Thanks for all the tips. I have just checked the suction from the manifold and that is present, the problem is obviously in the servo itself. Rather than strip the unit down I have splashed out on a NEW unit available for a reasonable price from HG Hive so I am going for that. Will post on how the fitting proceeds.

Thanks
Stuart
si robathan

Is this an old thread renewed...It was probably the diaphragm inside the unit.
/Moss
Moss

As a postscript, I have had a new servo fitted, this makes all the difference in the world to the cars breaking ability.
thanks for the advice.
si robathan

This thread was discussed between 17/03/2011 and 01/05/2011

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