Brake Master Cylinder: Adjustments

Proper adjustment of the master cylinder pushrod is necessary to ensure proper operation of the power brake system. A pushrod that is too long will cause the master cylinder piston to close off the compensating port, preventing hydraulic pressure from being released and resulting in brake drag. A pushrod that is too short will cause excessive brake pedal travel and cause groaning noises to come from the booster when the brakes are applied. A properly adjusted pushrod that remains assembled to the booster with which it was matched during production should not require service adjustment. However, if the booster, master cylinder or pushrod are serviced, the pushrod may require adjustment.
There are two methods that can be used to check for proper pushrod length and installation: the gauge method and air method. Usually, if the power unit pushrod requires adjustment, use the power unit repair kit gauge. The gauge measures from the end of the pushrod to the power unit shell.

GAUGE METHOD

Master Cylinder Pushrod Adjustment. Delco-Moraine Type Vacuum Booster:

Delco-Moraine Type
On these models, the master cylinder pushrod length is fixed and is usually only checked after the unit has been overhauled.
1. Assemble booster unit and install pushrod, ensuring pushrod is fully seated.
2. Position go/no go gauge furnished in repair kit over pushrod as shown in Fig. 1
3. If pushrod height is not within limits of gauge, install service adjustable pushrod and adjust rod to obtain correct height.
4. Install power unit and check adjustment, ensuring master cylinder compensating port is open with engine running and brake pedal released.

Fig. 2 Master Cylinder Adjustment Tool:

Fig. 3 Booster Piston Rod Adjustment:

Single Diaphragm Type
This unit is used on late model GEO Metro & Spectrum. The length of the booster piston rod is adjusted to provide specified clearance between the piston rod end and master cylinder piston. Before making an adjustment, push piston rod several times to ensure reaction disc is in place. Ensure gasket is installed to master cylinder and keep inside of booster at atmospheric pressure.
1. Place booster pin rod gauge No. J-34873 or equivalent on master cylinder and push pin until it contacts piston, Fig. 2.
2. Turn tool upside down and place it on booster. Adjust booster piston rod length until rod end contacts pin head.
3. Adjust clearance by turning adjusting bolt of piston rod, Fig. 3. If negative pressure is applied to booster with engine at idle, piston to piston rod clearance should measure 0.004-0.020 inch.

AIR METHOD

1. Be sure master cylinder attaching nuts are tight.
2. Remove master cylinder filler cap.
3. With brake released, force compressed air into the hydraulic outlet of the master cylinder. Regulate air pressure to a value of approximately 5 psi, to prevent spraying brake fluid from master cylinder. Care must be taken not to allow brake fluid to contact painted surfaces of vehicle, skin or eyes, as damage or personal injury will result.
4. If air passes through the compensating port, which is the smaller of the two holes in the bottom of the master cylinder reservoir, the adjustment is satisfactory.
5. If air does not flow through the compensating port, adjust the pushrod as required, either by means of the adjustment screw (if provided) or by adding shims between the master cylinder and power unit shell until the air flows freely.
6. Reconnect brake lines and bleed system.