1991 Ford Thunderbird Brake Service (ONLINE REPAIR MANUALS)

The vacuum assist diaphragm assembly multiplies the force exerted on the master cylinder piston in order to increase the hydraulic pressure delivered to the wheel cylinders while decreasing the effort necessary to obtain acceptable stopping performance.

Vacuum assist units get their energy by opposing engine vacuum to atmospheric pressure. A piston, cylinder and flexible diaphragm utilize this energy to provide brake assistance. The diaphragm is balanced with engine vacuum until the brake pedal is de...

ACTUATION ASSEMBLY

Refer toFigs. and , during the following procedure.

• With hydraulic system discharged, disconnect battery ground cable.
• Remove air cleaner housing and duct assembly, if necessary.
• Label, then disconnect electrical connectors from fluid level indicator, main valve, solenoid valve block, pressure warning switch, hydraulic pump motor and ground connector from master cylinder portion of actuation assembly.
• Disconnect and plug brake tube fittings. Do not allow brake fluid to come in contact with any electrical connectors.
• Remove trim panel under steering column. Di...

Remove wheels and inspect brake disc, caliper and linings. The wheel bearings should be inspected at this time and repacked if necessary. Do not get any grease on the linings.

If the caliper is cracked or fluid leakage through the casting is evident, it must be replaced as a unit.

Should it become necessary to remove the caliper for installation of new parts, clean all parts in alcohol, wipe dry using lint free cloths. Using an air hose, blow out drilled passages and bores. Check dust boots for punctures or tears. If punctures or tears are evident, new boots should be installed upon re...

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 prevent the master cylinder piston from completely releasing hydraulic pressure, eventually, causing the brakes to 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 is was matched during production should not require service adjustment. However, if the ...

• Remove wheel, drum and brake shoes.
• Disconnect hydraulic line at wheel cylinder. Do not pull metal line away from cylinder as the cylinder connection will bend metal line and make installation difficult. Line will separate from cylinder when cylinder is moved away from brake backing plate.
• Remove screws holding cylinder to brake plate and remove cylinder.

OVERHAUL

• Referring toFig. as a guide, remove boots, pistons, springs and cups from cylinder.
• Place all parts, except cylinder casting in clean brake fluid. Wipe cylinder walls with clean brake fluid.
• Examine cylinder bore. A scored ...

Complaints about power brake operation should be handled as if two separate systems exist. Check for faults in the hydraulic system first. If it is satisfactory, start inspecting the power brake circuit. For a quick check of proper power unit operation, press the brake pedal firmly and then start the engine. The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position.

Another check begins with installation of a suitable pressure gauge in the brake hydraulic system. Take a reading with the engine off and the power unit not opera...