The brake servo uses engine vacuum to amplify pedal force, making a significant difference to braking effort in town use, during repeated braking, and on longer journeys. The MGB was not originally fitted with a servo, it became available as a factory optional extra from February 1970 on cars with single-line braking systems only, and was not therefore available on North American dual-circuit cars at that time. An improved unit with a larger bore was fitted from May 1970. The V8 had a servo as standard from the outset.
It became standard equipment on four-cylinder home market cars from August 1973.
Servo Types
Three distinct servo configurations were used across the production run. On cars with single-circuit brakes, a single remote Lockheed servo was mounted at the rear of the engine compartment, on the side opposite the master cylinder pedal box. On North American cars with dual-circuit brakes, two servo units were fitted, the rear one serving the rear brakes and the front one mounted adjacent to the oil filter serving the front brakes.
From May 1977, when dual-circuit brakes were introduced for the UK home market, a direct-acting servo was combined with the master cylinder in a single inline unit, replacing the separate remote servo arrangement entirely.
Servo Kits & Conversions
For cars not originally fitted with a servo, a complete conversion kit is available comprising an OEM servo with a 1.65:1 assistance ratio, stainless steel mounting bracket, take-off adaptor, vacuum hose, brake line extension, and all fittings. A universal kit offering a higher 1.9:1 ratio is also available as an accessible alternative. Both suit cars with single-circuit brakes and require no modification to the master cylinder or pedal box. The stainless steel bracket kit is also available separately and gives a smart under-bonnet appearance compared to the standard painted steel bracket.
Single-circuit servos for cars to 1976 are available as reproduction, genuine OE, and for customer reconditioning. Dual-circuit direct-acting servos for 1977-onwards are available as new and for reconditioning.
Servo Components
Servo air filter and cover kits, non-return valves for both the servo body and the inlet manifold connection, vacuum hoses sold by the metre, air valve hoses, hose clips, and all copper crush washers and fixings are available throughout. The non-return valve on the servo body prevents vacuum loss when the throttle is opened, while the manifold valve prevents oil contamination of the servo diaphragm, both should be checked during any brake system service.