Ford Mid-Size Cars 1971-1985 Repair Guide

Power Brake Booster


See Figure 1

Power brakes operate just as non-power brake systems except in the actuation of the master cylinder pistons. On systems other than Hydro-Boost, a vacuum diaphragm is located on the front of the master cylinder; this device assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.

The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum, brake assist vacuum will not be lost.

Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder piston(s) to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm, and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder actuating rod, and direct mechanical application will occur as the pedal is depressed.


The hydraulic and mechanical problems that apply to conventional brake systems also apply to power brakes, and should be checked for if the tests below do not reveal the problem.

System Vacuum Leak
  1. Operate the engine at idle without touching the brake pedal for at least one minute.
  3. Turn off the engine, and wait one minute.
  5. Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. Light application will produce less and less pedal travel, if vacuum was present. If there is no vacuum, air is leaking into the system somewhere.

Proper System Operation

  1. Pump the brake pedal (with engine off) until the supply vacuum is entirely gone.
  3. Put a light, steady pressure on the pedal.
  5. Start the engine, and operate it at idle. If the system is operating, the brake pedal should fall toward the floor if constant pressure is maintained on the pedal.
  7. Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.

Most mid-sized models have been equipped with power brakes. On all drum brake equipped car, as well as those equipped with a disc front/drum rear brake configuration, the power assist has been supplied by a manifold vacuum-operated servo, located between the master cylinder and the firewall.

Click image to see an enlarged view

Fig. Fig. 1: Exploded view of master cylinder and pedal assembly-power system


Vacuum Booster

The following procedure can be used on all years and models. Slight variations may occur but the basic procedure should cover all years and models.

  1. Disconnect the negative battery cable.
  3. Disconnect the manifold vacuum hose from the booster check valve.
  5. Disconnect the brake lines from the master cylinder, remove the master cylinder-to-booster retaining nuts and remove the master cylinder.

On some vehicles, it may be possible to remove the master cylinder and move it aside without disconnecting the brake lines. If you do this, be careful not to kink the brake lines.

  1. Working inside the vehicle below the instrument panel, remove the stop light switch connector. Remove the switch retaining pin and slide the switch off the brake pedal pin just far enough for the outer arm to clear the pin, then remove the switch. Be careful not to damage the switch.
  3. Remove the booster-to-dash panel attaching nuts. If necessary, remove the cowl top intrusion bolt.
  5. Slide the booster pushrod, washers and bushing off the brake pedal pin. Remove the booster assembly.
  7. Installation is the reverse of the removal procedure. Tighten the booster-to-dash panel attaching nuts and the master cylinder attaching nuts to 13-25 ft. lbs. (18-34 Nm). If the brake lines were disconnected, bleed the brake system. Refer to the necessary service procedures in this section.

Hydro-Boost Hydraulic Booster

See Figure 2

The Hydro-Boost assembly contains a valve which controls pump pressure while braking, a lever to control the position of the valve, and a boost piston to provide the force to operate a conventional master cylinder attached to the front of the booster. The Hydro-Boost also has a reserve system, designed to store sufficient pressurized fluid to provide at least 2 brake applications in the event of insufficient fluid flow from the power steering pump. The brakes can also be applied unassisted if the reserve system is depleted.

Before removing the Hydro-Boost, discharge the accumulator by making several brake applications until a hard pedal is felt.

  1. Working from inside the vehicle, below the instrument panel, disconnect the pushrod from the brake pedal.
  3. Unfasten the stop light switch wires at the connector.
  5. Remove the hairpin retainer. Slide the stop light switch off the brake pedal far enough for the switch outer hole to clear the pin.
  7. Remove the switch from the pin. Slide the pushrod, nylon washers and bushing off the brake pedal pin.
  9. Open the hood and remove the nuts attaching the master cylinder to the Hydro-Boost. Remove the master cylinder. Secure it to one side without disturbing the hydraulic lines.
  11. Disconnect the pressure, steering gear and return lines from the booster. Plug the lines to prevent the entry of dirt.
  13. Remove the nuts attaching the Hydro-Boost. Remove the booster from the firewall, sliding the pushrod link out of the engine side of the firewall.
  15. Install the Hydro-Boost on the firewall and install the attaching nuts.
  17. Install the master cylinder on the booster.
  19. Connect the pressure, steering gear and return lines to the booster.
  21. Working below the instrument panel, install the nylon washer, booster pushrod and bushing on the brake pedal pin.
  23. Install the switch so that it straddles the pushrod with the switch slot on the pedal pin and the switch outer hole just clearing the pin. Slide the switch completely onto the pin and install the nylon washer.
  25. Attach these parts with the hairpin retainer. Connect the stop light switch wires and install the wires in the retaining clip.
  27. Remove the coil high tension wire so that the engine will not start. Fill the power steering pump and engage the starter. Apply the brakes with a pumping action. Do not turn the steering wheel until air has been bled from the booster.
  29. Check the fluid level and add as required. Start the engine and apply the brakes, checking for leaks. Cycle the steering wheel.
  31. If a whine-type noise is heard, suspect fluid aeration. Bleed the system.

Click image to see an enlarged view

Fig. Fig. 2: Hydro-Boost assembly and related parts