The MGB GT V8 uses a single-circuit brake master cylinder mounted on the bulkhead at the brake pedal. The master cylinder converts brake pedal force into hydraulic pressure, which is then routed through the brake pipework to the remotely-mounted Lockheed servo (which provides the vacuum-assist boost) and onward to the front calipers and rear wheel cylinders. The single-circuit specification, one hydraulic circuit serving both front and rear brakes, was the V8 factory standard throughout production.
Master Cylinder Operation
The master cylinder contains a piston in a precision-bored body, with seals on the piston that displace fluid as the piston moves forward. The fluid reservoir at the top of the cylinder maintains a fluid supply via compensating ports that are uncovered when the brakes are released and covered when the brake pedal is pressed. The reservoir holds enough fluid to compensate for normal lining wear and minor system seepage.
Service Kits
Master cylinder service kits include the rubber seals, return spring, push-rod boot and small fixings needed for a routine service. Internal corrosion of the master cylinder bore, a common consequence of moisture absorption by old brake fluid, usually means complete master cylinder replacement is required rather than service. Cylinder bore inspection should be part of any service procedure.
Reservoir and Cap
The V8 uses a separate brake reservoir distinct from the clutch master cylinder reservoir throughout production. The reservoir cap includes a vent that allows atmospheric pressure to act on the fluid surface as fluid is drawn out by lining wear; the vent must be clear or the master cylinder pressure-balance is disturbed.
Service Considerations
Master cylinder work should always be followed by complete brake system bleeding to remove air from the hydraulic circuit. Air in the system causes a soft, spongy brake pedal and reduced braking effectiveness. The bleeding procedure must follow the correct sequence to avoid trapping air in any part of the system.
MGOC Spares
MGOC Spares supplies V8 brake master cylinders, service kits, reservoirs and reservoir caps for the MGB GT V8.