Problem: Spongy or low brake pedal
Cause: If master cylinder is mounted at an angle air could be trapped in the high points if the brake line outlets are not at the highest point.
The secondary outlet of the master cylinder shown in Figure 80.1 is not at the end of the bore. This presents no problem if the master cylinder is mounted level. If this master cylinder is mounted at even the slightest angle air will be trapped in the end of the bore.
Figure 80.2 shows a GM 4 outlet QTU master. This master cylinder incorporates the pressure differential switch into the master cylinder. The access plug threads become the highest point in the master cylinder. This master cylinder is mounted at a relatively sharp angle. If air enters the left front or right rear wheel circuits it can migrate to the high point.
If the vehicle is experiencing a low and/ or spongy brake pedal and the master cylinder is mounted at an angle, trapped air might be the cause. use the steps below to check for trapped air.
Master cylinder mounted at small angle (Figure 80.3)
1. Raise the rear of the vehicle (Figure 80.4) to bring the master cylinder to a level position as shown in Figure 80.5.
Figure 80.4 Figure 80.5
CAUTION: Do not raise the vehicle in an unsafe manner.
2. If foot bleeding, crack the line fitting on the secondary brake line and have someone depress the brake pedal making sure not exceed 3/4 pedal travel. Repeat this process until no air is seen.
3. Repeat step 2 on the primary brake line. If pedal height and feel are restored repair is complete. If problem is still present see page 67 for additional diagnostic information.
Master cylinders mounted at sharp angles:
The steps above cannot be used to check for air in this type of master cylinder. Raising the rear end of the vehicle to a point where the master cylinder is level would create an unsafe condition. Use the steps below.
Method 1: (use if pressure bleeding)
1. With the cap adapter and pressure bleeder connected unbolt the master cylinder from the vacuum booster. Gently position the master cylinder so the secondary end of the master cylinder is slightly lower than the primary side. (Figure 80.6)
2. Connect a capture container to the left front wheel (or the wheel(s) supplied by the secondary outlet). Open the bleeder screw and let fluid flow.
3. As fluid flows tap on secondary end of master cylinder to help dislodge trapped air.
Method 2: Bleeding Master at Sharp Angle
1. Drain and clean reservoir where possible. Add enough fluid to cover ports (approximately 1/4″).
2. Unbolt master cylinder from vacuum booster. Gently change the angle of the master cylinder to make the secondary end (bumper end) to be lower than the primary end (firewall end) of the master cylinder (Figure 80.7).
3. Perform RFI on either wheel on the secondary hydraulic circuit using the correct procedures.
4. Reinstall master cylinder and adjust reservoir level to “Max”.