The MGB GT V8 was fitted with a vacuum brake servo as standard from the start of production, a key part of the V8 specification given the car's higher kerb weight and performance compared with the four-cylinder MGB. The servo provides power assistance to brake pedal effort, using engine inlet manifold vacuum to multiply the driver's pedal force into greater hydraulic pressure at the master cylinder. The system gives the V8 a reasonable pedal effort despite the greater stopping demands.
Servo Specification
The factory V8 servo is the GSM125 unit, a Lockheed non-direct-acting servo mounted on a bracket on the opposite side of the engine bay to the bulkhead-mounted master cylinder. The servo is a vacuum-operated diaphragm type, a large diaphragm inside the servo housing has engine vacuum on one side and atmospheric pressure on the other, with the pressure differential providing the assistance force. The servo has no power source other than engine vacuum, so it operates only when the engine is running. Brake fluid pipework runs from the master cylinder to the servo and from the servo to the brake circuits.
Mounting Brackets
The servo is mounted to the bodyshell via specific mounting brackets, BHH1148 (rear bracket) and BHH1149 (front bracket) in the factory specification, both finished in original black. The brackets locate the servo correctly on the opposite side of the engine bay to the master cylinder. Bracket integrity is essential, a loose servo causes spongy pedal feel and inconsistent braking. Stainless-steel replacement brackets are also available.
Vacuum Hoses and Non-Return Valves
A vacuum hose runs from the inlet manifold area on the engine to the servo, providing the vacuum supply that the servo needs to operate. A non-return valve in the hose prevents engine backfire pressures from reaching the servo and ensures the servo retains vacuum for several brake applications after the engine stops. The non-return valve and the hose itself are wear items, perished hose or a faulty valve causes sudden loss of servo assistance.
Servo Faults
Common servo faults include hardened vacuum diaphragm (causing reduced or absent assistance), failed non-return valve (causing instant loss of assistance when the engine stops), and external air leaks (causing reduced assistance and rough engine running due to the vacuum leak). A failed servo is dangerous, the brakes still function but require significantly higher pedal effort. Servo faults should be addressed promptly.
Service and Replacement
Brake servo service is specialist work; many servos are replaced as a unit rather than serviced. Servo kits with the components needed for diaphragm and valve renewal are available for some specifications. Complete replacement servos are available for the V8 application.
MGOC Spares
MGOC Spares supplies V8 brake servos, mounting brackets, vacuum hoses, non-return valves and servo service kits for the MGB GT V8.