The MGA With An Attitude

On 22 October 2017, Frank Watson in Western Australia, AU wrote:
"I have just fitted a brake servo to my MGA. When I start the engine the brakes are applied [locked-up] (not touching the brake peddle). Not sure what I have done wrong; any help would be appreciated.

First assure that the new servo unit is mounted in the correct orientation. The control valve assembly must be at the bottom, not HORIZONTAL, and the output end must be pointed somewhat upward, NOT level (see pictures). Orientation is important to allow air to escape from the servo unit when bleeding the hydraulic circuit.
Brake Booster Orientation
Brake Booster Connections Then be sure the pipe running from master cylinder to servo goes into the side of the servo, and the pipe from end of servo goes to the fluid splitter block. The servo will not work properly if these pipes are switched.

If the system is dysfunctional, there may be a fault in the servo unit. The servo can only put pressure into the system if either the air valve is open or there is a leak to atmosphere at the back of unit. The air valve is lifted to allow atmospheric pressure to the rear "can" of the unit by the movement of a small piston, under the diaphragm, being moved by the increase in hydraulic pressure when you the pedal is depressed. If it is opening the valve without the hydraulic pressure, that piston may be stuck open. This is quite common on old units (and maybe some faulty new units).

Try disconnecting the vacuum pipe and blanking off the manifold tap (or pinch the fat vacuum hose to close it off). This should solve the problem temporarily, show the fault and reverting to manual brakes (no power assist). Leaks can be past the air valve rubbers, a split rubber hose between the valve and rear can, a badly seated (not sealed) diaphragm between the two halves of the unit, or a slack compression band between the two halves (diaphragm not sealed).

See photos and notes form a tech session on brake booster rebuild.
See following page for possible failures and fault diagnosis.

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