The master cylinder and pedal-box assembly sits on a horizontal shelf behind the engine, above the footwell, with the pendant brake and clutch pedals hanging directly below the master cylinder and passing through a rubber fume excluder that seals the bulkhead. This is the single most variant-sensitive part of the MGA control-pedals area, because the master cylinder is specified differently depending on whether the car has a combined or separate brake-and-clutch hydraulic circuit. Pushrod-engined cars other than the De Luxe use a single combined master cylinder serving both circuits from a shared fluid reservoir, while Twin Cam and De Luxe cars use two separate master cylinders, one each for brake and clutch.
Combined Master Cylinder, Pushrod Cars
On all pushrod cars except the De Luxe, a single master cylinder with an integral supply tank operates both the brake and clutch circuits from a shared fluid reservoir, the body left as an unpainted casting from the factory with the supply tank cover in a natural metal finish. Two patterns exist, the 1500 type and the 1600 and 1600 Mk II type, which has a deeper cover over the supply tank introduced at the start of 1600 production.
Replacement combined master cylinders, supply tank covers, reservoir seals, rebuild kits, and mounting hardware are available for both patterns, the chassis number determining which pattern applies.
Separate Master Cylinders, Twin Cam & De Luxe
Twin Cam and De Luxe cars use separate master cylinders for the brake and clutch circuits, each with its own independent fluid reservoir, the brake master cylinder mounted turned on its side on the pedal-box shelf, a different orientation to the combined pushrod unit. During Twin Cam production the brake master cylinder assembly was revised, at car 997 the box being modified to remove the need for a taper packing between the master cylinder and its box, so earlier Twin Cam cars use the original box-with-taper-packing specification. The De Luxe variants use a slightly modified Twin Cam-pattern master cylinder rather than the standard pushrod unit, and a factory service kit was made available to convert an earlier Twin Cam master cylinder to the later De Luxe-specification arrangement.
Pedals, Rubbers & Fume Excluder
The pendant brake and clutch pedals pass through the pedal-box shelf below the master cylinder and exit into the footwell, the arms passing through a one-piece black rubber fume excluder that seals the bulkhead around the pedal apertures, this being of a different pattern on Twin Cam and De Luxe models from the standard pushrod item. The pedal pads are shield-shaped rubbers with vertical grooves, common to both pedals. Three pedal-arm patterns exist across production, an early right-hand drive 1500 type, a later right-hand drive 1500 and 1600 type, and a left-hand drive type, while the Twin Cam and De Luxe use a pedal where the arm extends further above the pivot point, though the rubber itself is common with the pushrod pattern.
Fluid Specification & Ordering
The pushrod combined master cylinder serves both circuits from a shared reservoir and was factory-filled with brake fluid to the specification current at the time of manufacture. Twin Cam and De Luxe cars with the Dunlop disc brake system were originally specified for Wakefield Crimson brake fluid, later superseded by Dunlop disc brake fluid, or any fluid to SAE70.R1 or SAE70.R3 specification. Modern fluid choices today commonly include DOT 4 and silicone-based DOT 5 alternatives, and owners considering a change from original-specification fluid should confirm compatibility with all seals and cylinder materials, as mixing incompatible fluid types causes rapid seal deterioration. Ordering begins with the car variant, the combined master cylinder in 1500 or 1600 cover pattern for pushrod non-De Luxe cars, and separate brake and clutch master cylinders for Twin Cam and De Luxe cars, with pedal-arm pattern depending on drive orientation and chassis number.
When refitting a master cylinder or renewing its seals, the full set of seal-kit components, reservoir cap, mounting hardware, and bleed nipples should be ordered as a single package.