A brake servo dramatically reduces the pedal effort needed to apply firm braking, transforming the driving experience on any classic MG that left the factory without one. The servo uses engine vacuum to multiply the force applied by the driver's foot, so the pedal pressure needed to achieve a given braking effort is much lower than on an unboosted system, making a meaningful difference in town use, during repeated braking, and on longer journeys where driver fatigue is a factor, with the typical reduction in pedal effort being around 50%.
For owners running cars in modern traffic, where the firm, deliberate pedal pressure of an original unboosted system can be tiring, the addition of a servo is one of the most valuable handling-related upgrades available.
Servo Conversion Kits
The tailored servo kit gathers the components needed for a complete installation, the servo unit itself being a vacuum diaphragm housed in a steel chamber with internal valving that controls the assistance proportionally to the pedal input, supplied in either 1.65:1 assistance ratio matching the factory-fitted servo level on original servo-fitted cars, or 1.9:1 and 2.3:1 ratios for owners wanting maximum assistance. The kit includes a Lockheed or Girling OEM-specification servo, a stainless steel mounting bracket that gives a smart under-bonnet appearance compared to the standard painted steel original, the take-off adaptor that connects the vacuum line to the inlet manifold, the vacuum hose itself, brake line extensions, and all necessary fixings, designed as a straightforward bolt-on addition to the existing single-circuit brake system that requires no modification to the master cylinder or pedal box. A remote servo was available as a factory optional extra on the MGB from February 1970, becoming standard on UK home-market cars from August 1973, but many earlier cars were never fitted with one and these benefit most from the retrofit, while later dual-circuit cars from May 1977 onwards use a different direct-acting servo integrated with the master cylinder and are not suited to the single-circuit kit.
The Factory-Fitted Servo Application
Some applications in the range were fitted with a brake servo as standard from the factory and were designed around servo-assisted operation, the MGC being the principal example where the C-series engine's weight makes the car heavier than the four-cylinder cars and the Girling braking system was designed around servo assistance. On these cars the kit is for servicing or replacing a worn original rather than retrofitting, the pedal effort without a functioning servo being considerably higher than most drivers expect and a failed or leaking servo manifesting as a hard, high-effort pedal that requires significantly more force than normal to achieve the same braking effect, both tiring and potentially dangerous, as the driver may not apply sufficient force in an emergency stop. The vacuum non-return valve should be checked as part of any servo diagnosis, as a leaking valve allows the servo's vacuum reserve to bleed away when the engine is at low vacuum under load or at higher rpm, causing the pedal to harden during the driving conditions when braking is most likely to be needed.
Servo Service Kits
Brake servo service kits provide the components needed to renew the worn internals of a tired original servo, restoring performance without the cost of complete replacement, typically including the diaphragm which is the most common wear item, internal valves, seals, and the gaskets needed to reassemble the case. Over time the diaphragm rubber hardens, develops cracks, or perforates outright, and a failed diaphragm causes loss of servo assistance and an air leak that affects engine running, while the internal control valves can also wear and lead to loss of response or to the servo failing to release between brake applications. Service requires removing the servo from the car, splitting the case halves, removing the worn components, fitting the new components from the kit, and reassembly with attention to correct orientation of the diaphragm and valves, with specialist tools needed for some operations, and where the case is heavily corroded complete replacement is more practical. The vacuum hose between manifold and servo and the non-return valve age in service alongside the internal components and are best renewed together.
For owners running modified or competition cars the technical team is available to advise on the right specification for the application, as competition cars sometimes prefer the firmer pedal feel of an unboosted system where the modulation precision is more important than the reduction in pedal effort.