The hydraulic system is the foundation of the MGB’s braking performance, and its condition determines not only stopping power but safety. A soft or spongy pedal that has not responded to bleeding is one of the most common complaints, and deterioration within the hydraulic circuit, rather than a bleeding fault, is almost always the cause, particularly on cars that have sat unused for extended periods. The hydraulic specification changed significantly across the production run.
From 1962, all MGBs used a single-circuit brake master cylinder serving all four corners, originally with a combined master cylinder and brake fluid reservoir, but from car number 119500 in 1967 cars for France and Benelux had a separate transparent reservoir. Export market cars received dual-circuit brakes from 1968, North American MkII cars first, then Sweden and Germany for the 1970 model year, Norway from early 1971, Switzerland from August 1972, and the Benelux countries from September 1974. UK home market cars did not receive dual-circuit brakes until May 1977, at car numbers 436465 (GT) and 437181 (Roadster).
Brake Servo
A remote Lockheed type 6 brake servo became available as a factory optional extra in February 1970, fitted only to cars with single-line braking systems. An improved servo with a bigger bore was fitted from May 1970. The V8 had the servo as standard from the outset. It became standard on four-cylinder home market cars from August 1973.
On North American GTs, a direct-acting servo finally appeared in December 1974. UK cars continued with the non-direct-acting servo until May 1977 when dual-circuit brakes and the direct-acting servo were introduced for the home market.
Brake Fluid
DOT 4 and DOT 4.1 glycol-based fluids are the correct specification for road use. Both are hygroscopic, absorbing moisture over time through rubber hoses and seals, and as moisture content rises the boiling point drops, increasing the risk of vapour lock and promoting internal corrosion. Brake fluid should be changed at a maximum of two-yearly intervals regardless of mileage.