The brake master cylinder converts the driver's pedal effort into hydraulic pressure that acts on the front callipers and rear wheel cylinders. The master cylinder specification changed several times during Midget production, and the correct replacement depends on the chassis number and engine type.
Master Cylinder Variants
Four distinct brake master cylinder specifications were used across production. The original cup-type single-circuit cylinder was fitted to 1275cc and early 1500 models. This was superseded by a replacement type identified by two concentric rings on the body, which serves all 1275cc and early 1500 applications. From GAN6-200001 (August 1977), a revised master cylinder with a remote reservoir mounted separately on the inner wing was introduced.
From GAN6-212001 (May 1978) onward, a dual-circuit master cylinder was fitted, dividing the braking system into two independent hydraulic circuits, typically front and rear, so that if one circuit fails, the other retains some braking capability. Seal kits are available for all current-supply master cylinders, allowing internal seals to be renewed without replacing the entire unit.
Pedal Box
The master cylinder is housed in the pedal box on the bulkhead, sharing the housing with the clutch master cylinder. Two pedal box types were used across production. The pedal box gasket seals the box to the footwell panel and should be inspected whenever either master cylinder is removed, a failed gasket allows moisture to enter the pedal box cavity, corroding the internal pushrod mechanisms and the master cylinder mounting surfaces. The cover plate on the engine side of the pedal box is available in standard and stainless steel, the stainless version is a worthwhile upgrade during any pedal box overhaul, as the original mild steel plate corrodes heavily in the engine bay environment.
Pedal rubbers are a shared item between the brake and clutch pedals and should be renewed if worn smooth, as a polished rubber surface reduces grip in wet conditions.
Brake Light Switch
Two brake light switch types were used: an earlier hydraulic switch operated by fluid pressure in the brake circuit, and a later mechanical switch operated by the brake pedal movement. The correct type is determined by the production date and the pedal box configuration.
Bleeding the System
When fitting a new master cylinder, the system must be bled thoroughly to remove all trapped air. Bleeding should start from the wheel furthest from the master cylinder and work toward the nearest. The brake fluid specification is DOT 4, DOT 5 silicone fluid should not be used unless the entire system has been specifically converted, as it is incompatible with the original rubber seals and hoses.