The brake master cylinder converts pedal force into hydraulic pressure and is the most common cause of a soft or spongy pedal that does not respond to bleeding. Internal seal degradation, typically caused by corrosion within the bore on cars that have sat unused for extended periods, allows fluid to bypass the piston rather than build pressure. A pedal that firms up after pumping but will not hold pressure is the classic symptom of a failing master cylinder.
Single-Circuit Master Cylinders
From 1962, all MGBs used a single-circuit brake master cylinder serving all four corners, with a combined master cylinder and brake fluid reservoir. From car number 119500 in 1967, cars for France and the Benelux countries were fitted with a separate transparent brake fluid reservoir. Two single-circuit master cylinder types were used across the production run, the earlier type with no identification rings on the cylinder body, and the later type identifiable by two concentric rings. Two seal kit specifications correspond to these types: the earlier cup-type seal kit for the unmarked cylinder, and the later ring-type seal kit for the two-ring cylinder.
Dual-Circuit Master Cylinders
Export market cars received dual-circuit brakes from 1968, North American MkII cars first, with Sweden, Germany, Norway, Switzerland, and the Benelux countries following progressively through to 1974. UK home market cars did not receive dual-circuit brakes until May 1977, at car numbers 436465 (GT) and 437181 (roadster), when a tandem master cylinder with an integral direct-acting servo was introduced. The dual-circuit master cylinder and its seal kit are available in genuine OE and reproduction.
Seal Bedding Note
In recent years, a specific issue has been observed with new master cylinders and replacement seal kits. Modern EPDM synthetic piston seals can shrink back through lack of use, producing a soft pedal even on a freshly rebuilt system. The solution is to expose the seal to brake fluid under sustained pressure, achieved through firm repeated pedal application, or by bracing the pedal under pressure with a timber cut to length against the seat frame and leaving it for 24 hours. This simple procedure has resolved the issue consistently.
Pushrods, piston return springs in both early and late specifications, filler caps in original metal and later plastic, brake master cylinder shims, banjo connectors and sealing washers for both brake and clutch master cylinders, and the dual-circuit brake failure warning switch are all available throughout.