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MG MGB Technical - 80 B servo insanity

Greetings to the list,

After installing a new brake M/C and rebuilding the in-line servo late last Oct./01, I find I cannot discover where the vacuum leak in the servo system is coming from. I've become an expert at removing peddle boxes, M/C's, and servos by doing those tasks at numerious times. I've checked the anti return valves to the manifold, bled the brake lines, etc..

The symptoms are, when engaging the brake (peddle) the process starts as normal, car begins to slow, but the peddle collapses with a whooshing sound near or around the peddle box/servo/M-C. The brakes at that point make very positive contact stopping the car _very quickly_, but the brakes release when I remove my foot from the peddle, they do not lock. The engine also begins a very lumpy idle until the brake is released.

The area in the assembly I'm not sure is correct, is the cupped seal, and metal cup that fits from the servo to the M/C,there's a plastic washer that came with the servo rebuild kit that may be or not be in the right location. My ear tells me that is where the leak is coming from but I cannot be sure.

Is there anyone in the MG community that has a diagnostic method or personal experience with servo problems that can get me on the right track?

Any and all help will be gratefully accepted. Thank you in advance.

Regards,

Bob



Bob Holton

Ok, I'll try a crack at this.

I have rebuilt the servo on my 79B. I did not experence any air leak problems with the servo,but here are some thoughts.

Was the repair kit an AP lockheed product? Substitutes do not cut it for the B brake system. I still have have the orginal AP lockheed instructions if that will help.

As I understand it bleeding the brakes won't help an air leak in the servo.

Did you replace the hose between the manifold and the servo with a new one?

Did you grease up (silicon/rubber grease) the rubber cup seal (and the big diaphram) so it would have good seal?. The rubber cup goes over the back end of the MBC with the metal cup going over it.

With the rod that goes from the servo to the back of the MBC, it has a disk that it rests on in the servo. Did you make sure that disk was in place in the servo?

Did you check the check valve on the manifold as well as the one on the servo? (The late B has two types of check valves for the maifold attachment. One is a pure two way adapter, the other type is a true check valve. A screwed up check valve on the manifold causes a "stone" pedal, i.e. air pressure is maintained in the servo.)

John Long
John D. Long

This thread was discussed between 07/05/2002 and 08/05/2002

MG MGB Technical index

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