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MG MGB Technical - Brake Booster

Since the brake booster overhaul on my 1969 MGB[single line system]the front shakes violently under braking at approx. 60mph when the booster is disconnected braking is smooth at all speeds any suggestions/experience?
Trevor

It is likely that the control valves in the booster are not operating properly causing it to pulse off and on. Which booster are you using?
J Heisenfeldt

Brake booster is aGirling Mk2B[ex MGC]
Trevor

I happen to have an MCG. First rebleed the system, an air bubble in the bore of the control piston may be allowing it to oscillate, as it requires fluid pressure from both sides to operate properly, if there is an air bubble in either the control piston bore or the main piston bore it could be causing your problem.If no change check for leakage of fluid, remove the non return valve (the one with the vacuum line connected) and stick your finger in and check for brake fluid in the bottom of the vacuum canister. Remove the four screws holding the cover on the valve chest (this is the small cover with a tube running back to the the vacuum canister) look for any leaking fluid in the valve chamber. Check the two valves in the valve chamber, they are small diameter plastic pads mounted on a rocking T shaped piece that is held in by two screews. With no pressure on the brake pedal the valve towards the front of the car should be seated against the aluminum port. Have someon step on the brake pedal, the T peice should rock so that the front valve opens and the rear one seats on the aluminum port. If you find fluid in either of the two places the inner seals on the main or contol piston are leaking and the booster will need to be taken apart. If you have no leak, one of the two valve pads may be leaking air or vacuum. The rear valve pad contols the vacuum level on the rear of the diaphram in the vacuum chamber. The front pad allows air into the rear of the vacuum chamber. When the brake pedal is not being pressed the air valve is closed and equal vacuum is applied to both sides of the diaphram in the chamber, so it does nothing. When the brake is pressed the internal control piston will move forward, this rocks the T piece and closes the vacuum port to the front section of the diaphram. At the same time the front pad opens to let air in to the rear of the diaphram so that the vacuum pulls it forward and applies pressure to the brakes. In order to chatter, the most likely problem is that the control piston seals are leaking in the bore causing it to oscillate.
John H

John,many thanks the info ,would you suggest brake bleeding with the engine running i.e vacuum on the booster?
Trevor

Running the engine will have no effect on bleeding. They can be hard to bleed due to the brake lines comming in from the top. Have someone put heavy pressure on the pedal, then open the bleeder, the rush of fluid will help carry small air bubbles out.
John H

This thread was discussed between 12/06/2008 and 16/06/2008

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