What is it that usually causes the brake pedal to feel spongy and to offer adequate resistance only when pressed several times in a row (pumping)?

Question

DentroTest · Accepted Answer

Brake fluid is an incompressible fluid, which makes it possible to transmit the force of the pedal to the braking elements. Air, on the other hand, is a compressible gas: if bubbles get into the passages of the hydraulic circuit, when the brake is pressed that air is compressed instead of moving the pistons, and the pedal becomes soft and spongy. When pumping, the air is compressed temporarily and pressure builds up, so the pedal offers more resistance. The solution is to bleed the circuit at a workshop. A brake booster fault does not produce that symptom: the pedal would be very hard from the moment it is pressed, without softening or improving when pumped. And wear of pads or shoes tends to increase the initial travel of the pedal, but does not make it regain resistance when pumped.

What is it that usually causes the brake pedal to feel spongy and to offer adequate resistance only when pressed several times in a row (pumping)?

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