GM Firebird 1967-1981 Repair Guide

Firebird System Description


See Figures 1, 2 and 3

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Fig. Fig. 1: Internal workings of a common distribution and switch assembly

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Fig. Fig. 2: Cutaway view of a typical combination valve used on vehicles equipped with front disc brakes

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Fig. Fig. 3: Parts of a typical brake master cylinder used on all Firebirds

All Firebirds are equipped with two separate and independent brake systems, a front brake system and a rear brake system. This independence originates at the master cylinder. The master cylinder, mounted on the firewall, consists of two separate fluid reservoirs. They are independent of one another; fluid cannot pass from one to the other although they are contained within the same housing. The front reservoir supplies fluid to the front brakes only while the rear reservoir and outlet connects to the rear brakes. If a leak developed in either system, front or rear, the fluid level in the good system would provide enough pressure to stop the car. Situated below the reservoirs is a cylinder, housing a primary and a secondary piston and springs. Fluid from the reservoirs drains down into this cylinder and the pistons mechanically activated by the brake pedal exert pressure on the fluid transmitting it through the brake lines to the wheel cylinders at each wheel. The fluid pressure then enters each wheel cylinder or caliper (disc brakes) and forces the pistons outward. These pistons are linked to the brake shoes or, as with disc brakes, contact the brake pads. When the pistons move, so do the shoes or pads. The pads contact a round flat disc and the shoes move outward to contact a round metal drum surrounding them. The disc or drum is attached to the revolving wheel and the friction of the lining or pads against it slows the revolving wheel to a stop. With use, brake linings and pads gradually wear down and, if not replaced in time, their metal support plates (shoes) are exposed to contact and damage the drum or disc. The pad or lining surface attached to the metal shoe wears away and, eventually, braking ability decreases to a point requiring shoe and lining or pad replacement.

Self-adjusting drum brakes were standard equipment on all 1967-69 Firebirds. Front disc brakes were optional during these three years but became standard equipment on 1970 and later models. The drum brakes are the Duo-Servo, single-anchor (the brake shoes pivot on one anchor pin) type having bonded brake linings; that is, the linings are bonded rather than riveted to the metal shoes. The wheel cylinders each contain two pistons. Dirt and moisture are kept out of the cylinder by a rubber boot positioned over each end. The wheel cylinders are nonadjustable.

Beginning in 1967, all cars were equipped with a brake pipe distribution and switch assembly mounted below the master cylinder. This assembly is approximately rectangular; attached to the top are two brake lines (one for the front system and one for the rear) coming from the master cylinder. Fluid comes from the master cylinder, passes through this assembly, and exits to the wheels through two outlet lines mounted at each end of the assembly. An electrical switch is situated at top center on the assembly. At the top of the switch is a wire going to the parking brake light on the instrument panel and at the bottom of the switch (within the assembly) is a retractable pin which, when contacted, activates the light on the dash. Directly below the switch pin and running the length of the assembly is a passageway, or cylinder, housing a piston. Connected to each end of this cylinder is the fluid outlet line going to the wheels. Fluid exits the master cylinder through its two outlet lines and then passes through the distribution assembly and out the outlet lines to the wheels. As long as no leaks exist within the system, fluid passes through the inlet ports and flows into the passageway containing the piston. This fluid exerts an equal pressure at each end of the piston thereby keeping the piston centered in one location. The piston is flat along its top except for a depression or dip in the center. When no leaks exist, this dip is directly beneath the electrical switch activator pin. If a leak develops within one of the systems (front or rear), fluid pressure at one end of the piston drops and the piston is forced to one end of its cylinder. The piston moves off center and the elevated area of the piston now contacts the switch pin and activates the light on the dash, warning the driver that a leak has occurred. Brake pedal free-play will increase but the remaining system (front or rear) will stop the car. When the leaking system is repaired and the lines are bled of any air, the piston will return to its central position and the switch will be reset to the off position causing the light to go out. This light will continue to go on, but only when the parking brake is applied.

From 1967 to 1970, all Firebirds equipped with air conditioning, an 8 7 / 8 in. rear axle ring gear (1969), or disc brakes, have a pressure regulator valve (called a metering valve on later disc brake cars) mounted on the left frame side rail (drum brake cars) or beneath the master cylinder (disc brake cars). On drum brake cars, this valve operates off the rear brake line but on disc brake cars, it is connected to the front brake line. This valve controls the hydraulic pressure to the rear brakes (drum brake cars) or to the front brakes (disc brake cars) so that front and rear brakes apply at the same time. It guards against early lock-up of the front or rear wheels during brake application. On disc brake cars, a pin on the end of this valve (underneath rubber boot) must be held down (this allows the valve to remain open) during bleeding operations. To depress the pin, press in on the rubber boot.

Beginning in 1970, front disc brakes became standard equipment on the Firebird. Valving included the brake pipe distribution and switch assembly and the metering valve described earlier. Beginning in 1971, all cars were equipped with a combination valve mounted below the master cylinder. This valve is the combination of the brake failure warning switch, metering valve, and the proportioning valve, all in one assembly. This valve is nonadjustable and non-serviceable; it must be replaced when defective. The proportioning valve (new in 1971) prevents early rear wheel lockup during quick, "panic" stops. Brake line fluid pressure is permitted to increase up to a certain point. When this point is reached, the valve begins to limit the amount of pressure going to the rear brakes. This prevents the rear brakes from locking up before the front disc brakes. If a leak occurs in the front system, a bypass within this valve sends full pressure to the rear brakes. This is especially effective during a quick, "panic" stop in a light car. A sudden stop in a light car causes the front end to dip, resulting in weight transfer to the front, making the rear end even lighter and more susceptible to sliding. The proportioning valve does not work during normal stops. When bleeding the brakes, the combination valve must be held in the open position by pressing in on the pin at the end of the valve.

As mentioned earlier, front disc brakes were available only as an option from 1967 to 1969. The 1967 and 1968 brakes were of the four-piston, fixed-caliper type; in 1969 a change was made to a new single-piston, sliding-caliper type. In 1970, the sliding-caliper type became standard front wheel equipment and remained unchanged for later models. The fixed-caliper type consists of the caliper, a rotating disc, a splash shield, and a mounting bracket. The caliper contains four pistons and two brake shoes (pads) with riveted linings. Behind each piston is a spring; assembled with each piston is a rubber seal and dust boot so that fluid can be kept in, while keeping dirt and moisture out. A retaining pin, secured by a cotter pin, passes through each caliper half and both shoes to hold everything together. One caliper half with brake shoe is mounted to the inside of the disc while the other half is mounted to the outside of the disc. As the brakes are applied, fluid pressure behind each piston forces the pistons outward against each pad (shoe); each pad then contacts a side of the rotating disc and the friction created stops the car. The sliding-caliper type consists of a one-piece housing, bored on the inboard side for a single piston. A rubber seal within this bore retains fluid and prevents it from seeping between the piston and the cylinder. Stepping on the brake pedal causes fluid pressure to force the piston and shoe outward to contact the inboard surface of the disc. This causes the caliper to slide inward on four bushings and two bolts, thereby forcing the outboard pad (shoe and lining) against the outer surface of the disc.

The Duo-Servo drum brakes are used on all cars with front disc brakes. Rear brake design has remained unchanged since 1967. Power brakes have been available as an option on all Firebirds. This type of brake reduces pedal effort through the use of engine vacuum.