I always understood that a restrictor valve in the brake circuit was to
reduce pressure to a particular part of the system to prevent lock-up under
heavy braking.
The front pads are mounted in brake calipers which each have two pistons.
Each piston has a rubber seal around its circumference within its housing.
Deformation of this seal is why the pads are always in close contact with
the disc (rotor) but not touching.
Basically under pressure the seal stretches, when the brake pedal pressure
is released the seal regains its shape and pulls the piston back a small
amount. Ingenious but it works.
I believe that TRs were the first production car to have disc brakes fitted
as standard.
Adrian TR4A CT64306
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