Problem: Spongy brake pedal
Cause: While there are many causes of a spongy brake pedal, air in the hydraulic system is one of the most common. If there is air anywhere in the hydraulic system it will be compressed as the brake pedal is applied resulting in the "spongy" pedal feel.
Solution: Often times one of the biggest challenges of getting the air out is finding out where it is. If you happen to be working on a vehicle with an "open faced reservoir" master cylinder there is a trick you can use to help find the air. "Open faced reservoir" masters are the one where you can see the ports for both circuits as shown in Figure 72.1.
Figure 72.1If you suspect air in this type of system follow these steps to find out which circuit it is in.
1. Drain the reservoir to a point where there is approximately 1/2" of fluid left in each reservoir.
2. Put the cap on the master cylinder reservoir. Have someone pump the brake pedal up to build pressure in the system.
3. With the brake pedal being held in an applied position, remove the cap from the master cylinder reservoir.
4. While watching the vent ports have the person in the vehicle release the brake pedal quickly.
5. If there is air in either the primary or secondary circuit it will force brake fluid through the vent port on release of the brake pedal as it expands. The pumping of the pedal caused the air to be compressed and holding the pedal in the applied position kept it compressed. Releasing the brake pedal allowed the air to expand and the expansion displaces fluid. If there is a "geyser" created on brake release that is the circuit with the air.
Figure 72.2
