Genuine AP Lockheed boxed, made in the UK.
The front-to-rear master cylinder is the latest dual master cylinder model, introduced in 1985, featuring a stepped bore of 17.8 to 19mm to replace previous units from September 1978 until 1988, when the built-in servo-type master cylinder was introduced. A key feature is the failure switch on the reservoir cap, identified by a yellow tab. This replaces all earlier square-type reservoir cylinders, such as GMC173/198 and GMC192/195. Note that this master cylinder has two different-sized metric unions: the top one is 10mm, and the bottom one is 12mm, suitable for the larger stepped bore on the rear brakes.
New pipe kits are required—BAU5655MS for left-hand drive and BAU5654 (or BAU5654MS in copper-nickel) for right-hand drive vehicles.
Vehicles manufactured after late 1988 feature a built-in servo, reference GMC90376.
The bottom piston operates the rear brakes, while the upper piston controls the front brakes; earlier models may have the pistons configured oppositely.
Use only DOT4 or DOT5.1 brake fluid, as these cylinders are not compatible with racing fluids like Castrol SRF or mineral-based fluids.
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