Description & Operation

NOTE: Unless otherwise specified, references to Pickup include the F350 Super Duty commercial chassis.

NOTE: For any problems with abbreviations please see the ABBREVIATIONS article in the GENERAL INFORMATION Section.

MODEL IDENTIFICATION

Series ⁽¹⁾

Model

"A"

Aerostar

"E"

RWD Van

"F"

Pickup

"R"

Ranger

"U"

Bronco & Explorer

⁽¹⁾	Vehicle series is fifth character of VIN.

Front disc and rear drum brakes are standard on all models except "F" Series Super Duty, which comes with front and rear disc brakes. A single anchor, dual piston brake assembly is used on E250/350, F250/350 and front and rear on "F" Series Super Duty. All other models use a single piston, pin rail sliding caliper front disc brake system. Rear drum brake assembly consists of a support plate, 2 brake shoes, return springs, automatic adjuster components and one dual piston wheel cylinder. On Aerostar, Explorer and Ranger, brake drums are available in 9" and 10" sizes.

On rear drum brake models, parking brake is actuated by a cable, which pulls the parking brake lever located inside rear brake assembly. The lever pivots parking brake strut between brake shoes. This movement pushes outward on brake shoes. The brake shoe linings are made of asbestos-free fiberglass material.

"F" Series Super Duty, equipped with rear disc brakes, uses a cable actuated, transmission mounted parking brake. Transmission mounted parking brake is situated on transmission extension housing, and consists of a case assembly and cable actuated 9" x 3" Bendix brake assembly. See Fig 1. Case assembly consists of roller bearings, companion flange and mainshaft, assembled in a one-piece aluminum housing. Case assembly has a lubrication supply separate from transmission.

NOTE: All Figures in this article are Courtesy of Ford Motor Co.

Fig 1: Locating Trans. Mounted Parking Brake ("F" Series Super Duty)

Courtesy of FORD MOTOR CO.

Aerostar is equipped with a cartridge master cylinder and vacuum booster. The cartridge-type master cylinder has, in addition to the standard primary and secondary pistons, a by-pass proportioning valve and differential pressure switch.

Bronco, Explorer, Ranger and "E" & "F" Series models are equipped with dual master cylinder and vacuum booster. The dual master cylinder has primary and secondary pistons and separate primary and secondary fluid reservoirs. "F" Series Super Duty model uses a dual master cylinder and hydro-boost. On all models, caliper is secured to steering knuckle by 2 caliper retaining pins. Aerostar, Explorer and Ranger use a dual-diaphragm vacuum booster; all others use single or dual-diaphragm vacuum booster.

The single and dual-diaphragm (tandem) vacuum boosters are self-contained, vacuum power brake units. The booster assists in actuating master cylinder push rod. Booster contains either a single or dual vacuum-suspended diaphragm which uses engine manifold vacuum or atmospheric pressure for power. Diesel powered vehicles use a belt-driven vacuum pump to provide vacuum for booster actuation.

A mechanically operated booster check valve controls power brake application and release in relation to foot pressure applied to check valve operating rod. Booster check valve is the only serviceable component of brake booster assembly. The "F" Series Super Duty hydro-boost is hydraulically operated by the power steering pump and provides a variable power assist regulated by brake pedal pressure.

Hydro-boost has a reserve system (compressed gas accumulator) which stores enough fluid under pressure to provide at least 2 power-assisted brake applications if power steering pump fluid flow stops. Brakes can also be operated manually if reserve system is lost.

Hydro-boost power units in normal condition produce certain noises. These noises occur when brakes are used more than usual. In general, these noises are hissing, clunks, clicks and chattering. Hydro-boost hisses when greater than normal braking effort is used. Hissing increases when pedal effort and operating temperatures increase. Loud hissing at normal pedal effort of 25-35 lbs. (11-16 kg.) or less should be investigated. Clunk, chatter and clicking noises are heard when brake pedal is quickly released from hard braking.

A fluid level indicator is located in the plastic reservoir of each master cylinder. It is not serviced separately. It consists of a float, magnet assembly and reed switch. The magnet operates the reed switch when fluid is low and activates a dash-mounted warning light.

WARNING: Hydro-boost unit should not be carried by accumulator. DO NOT drop hydro-boost on accumulator. Snap ring should be checked for proper seating before booster is used. Accumulator contains high pressure nitrogen gas and can be dangerous if mishandled. Before disposing of boosters, drill a 1/16" (1.6 mm) hole in end of accumulator can to relieve gas pressure.

The cartridge-type master cylinder on Aerostar uses a pressure differential warning switch. This switch lights brake warning light on dash if pressure in either front or rear brake system becomes too low.

Aerostar and some Bronco and "E" & "F" Series vehicles use a proportioning valve integral with master cylinder. See Fig 2. This proportioning valve has a by-pass feature. The proportioning valve restricts hydraulic pressure to the rear brakes to prevent lock-up. The by-pass feature allows full hydraulic pressure to the rear brakes in case of front brake system failure.

Fig 2: Examining Master Cylinder & Proportioning Valve (Bronco, "E" & "F" Series)

"F" Series Super Duty vehicles are equipped with a height sensing proportioning valve. See Fig 3. The height sensing proportioning valve monitors changes in vehicle height. This valve automatically provides optimum front-to-rear brake balance, regardless of vehicle load. The valve controls pressure to rear brakes by sensing vehicle load conditions through movement between rear axle and body. As vehicle load increases (resulting in decreased vehicle height), higher brake line pressure to rear brakes is allowed. The valve is located on No. 5 crossmember.

CAUTION: If height sensing proportioning valve linkage is disconnected, proper indexing will be lost. A new sensing valve must be installed. The new sensing valve will have the shaft pre-set and secured internally. If shaft turns freely, DO NOT USE.

Fig 3: Examining Height Sensing Proportioning Valve ("F" Series Super Duty)

CAUTION: Aftermarket load leveling kits, air shocks or modifications which change curb ride height and/or spring deflection rate provide false readings to height sensing proportioning valve. These modifications may result in unsatisfactory brake performance, which in turn could result in an accident.