Problem: Low spongy pedal as a result of trapped air in certain types of brake valves
Cause: The internal design and fluid flow through certain types of brake valves creates the possibility for air to become trapped. The trapped air will not be able to be removed using foot, pressure, vacuum or even RFI bleeding techniques.
Solution: These valves have to be “burped” to remove the trapped air. Typically technicians will attempt to bleed these valves by cracking the inlet and outlet lines. This may work on some types of valves but not on valves similar to the one shown in Figure 79.1. This is a combination valve used on certain Chrysler FWD vehicles. The valve is where the diagonal split takes place. Housed inside of the valve are a pressure differential switch and two proportioning valves, one for each rear wheel. On most vehicles the valve is located either on the firewall to the left of the master cylinder or the unibody frame rail just under the master cylinder (see Figure 79.2). The design of the proportioning valves allows air to become trapped around the proportioning valve return spring (see Figure 79.3). This air normally cannot be removed using conventional bleeding methods. If air becomes trapped it will cause a low spongy pedal and possibly a brake imbalance which can result in a premature pad wear condition.
The two proportioning valves are held in place by two 3/4″ retaining nuts. The air can be removed using these two nuts.
Figure 79.1 Figure 79.2
1. Start with the proportioning valve that is supplied by the secondary circuit of the master cylinder. Loosen the 3/4″ retaining nut until fluid flow is established.
2. With the nut loosened have an assistant depress the brake pedal. While the pedal is being depressed tap the valve body with a brass hammer or wooden hammer handle to dislodge any trapped air.
NOTE: If the valve is mounted at an angle the high side of the valve will be the most likely spot for trapped air.
3. With the pedal held in an applied position tighten the retaining nut. Repeat steps 1 & 2 until no air is seen during the bleeding.
4. Repeat the above steps with the primary circuit proportioning valve.
5. bleed at the wheels using the following manufacturers sequence, in this case RR, LF, LR, RF.
While the above procedures are specific tot he valve shown in Figure 79.2 they can be applied to valves of similar design.