Ford Pick-ups and Broncos 1987-1996 Repair Guide

F-Super Duty Hydro-Boost Brake Booster


See Figures 1, 2 and 3

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Fig. Fig. 1: Hydro-Boost system on the F-Super Duty chassis cab

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Fig. Fig. 2: Hydro-Boost system on the F-Super Duty commercial stripped chassis

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Fig. Fig. 3: Hydro-Boost system on the F-Super Duty motor home chassis

A hydraulically powered brake booster is used on the F-Super Duty truck. The power steering pump provides the fluid pressure to operate both the brake booster and the power steering gear.

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.


See Figure 4

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Fig. Fig. 4: Hydro-Boost installation

Do not depress the brake pedal with the master cylinder removed!

  1. Remove the master cylinder from the Hydro-Boost unit. Do not disconnect the brake lines from the master cylinder! Position the master cylinder out of the way.
  3. Disconnect the 3 hydraulic lines from the Hydro-Boost unit.
  5. Disconnect the pushrod from the brake pedal.
  7. Remove the booster mounting nuts and lift the booster from the firewall.

The booster should never be carried by the accumulator. The accumulator contains high pressure nitrogen and can be dangerous if mishandled! If the accumulator is to be disposed of, do not expose it to fire or other forms of incineration! Gas pressure can be relieved by drilling a1/16in. (1.5mm) hole in the end of the accumulator can. Always wear safety goggles during the drilling!

  1. Installation is the reverse of removal. Tighten the booster mounting nuts to 25 ft. lbs. (34 Nm); the master cylinder nuts to 25 ft. lbs. (34 Nm); connect the hydraulic lines, refill and bleed the booster as follows:
    1. Fill the pump reservoir with Dexron®II ATF.
    3. Disconnect the coil wires and crank the engine for several seconds.
    5. Check the fluid level and refill, if necessary.
    7. Connect the coil wires and start the engine.
    9. With the engine running, turn the steering wheel lock-to-lock twice. Shut off the engine.
    11. Depress the brake pedal several times to discharge the accumulator.
    13. Start the engine and repeat Step E.
    15. If foam appears in the reservoir, allow the foam to dissipate.
    17. Repeat Step E as often as necessary to expel all air from the system.


The system is, in effect, self-bleeding and normal vehicle operation will expel any further trapped air.