Disc brakes take very little fluid volume, but generally higher pressure. The piston does not retract much at all.
Drum brakes the piston is retracted after braking by the springs on the shoes, more movement if not adjusted regularly or properly.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-