There's a small port in the m/c that is uncovered by the first wiper seal on the piston.....if the piston does not return to uncover this unloader port...pressure builds up in the brake system and pads/shoes drag.

On 60's and 70's euro cars/trucks, there was a 'residual pressure valve' at each m/c outlet to maintain about 4 to 6 psi pressure in the system.

These had a habit of failing to bleed off pressure and would make the brake operation unstable when applied at speed.
Modern systems do not have this feature as it was primarily to preload the piston seals in the cylinders to prevent system collapse and keep umbrella seals open against the cylinder walls.

It also reduced m/c stroke to give a better pedal feel.


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