The design of the brake system, description of units and mechanisms

Scheme of the brake system of models with the location of pressure control valves on the GTZ assembly

Scheme of the brake system of models with the location of the pressure regulator valve at the rear axle (the electrical part of the system is similar to the one shown in the illustration above)

GTZ design

Pressure Control Valve Components

The design of the servo drive of the vacuum brake booster

Installation sockets of the servo drive of the vacuum booster of brakes

Brake caliper cylinder

Deformation of the piston sealing collar when the brake mechanism is actuated

Drum brake assembly components

Drum brake assembly components (continuation)

Section of a wheel cylinder

Note. This Section is devoted mainly to the description of the design of the brake systems of non-ABS equipped models. Information on the ABS device is given in Section Anti-lock brake system (ABS) - general information.

The functional diagram of the brake system is shown in the accompanying illustrations.

The system is actuated by the foot brake pedal. Through the vacuum booster servo, the force applied to the pedal is transmitted to the 2-circuit GTZ. The increase in pressure in the GTZ fluid is transmitted through hydraulic lines to the brake cylinders of the front and rear wheels of the car. One or two pressure control valves are included in the hydraulic circuit of the rear brakes.

The vacuum booster is connected to the inlet pipeline and is designed to increase the force applied to the foot brake pedal and the force exerted on the GTZ piston.

Regulator valves are used to limit the pressure of the hydraulic fluid in the circuit of the brake mechanisms of the rear wheels in order to prevent premature blocking of the latter during heavy braking. Note: ABS-equipped models do not need these restraints.

The pistons of the brake calipers of the front wheels act on the brake pads, which are tightly pressed by the friction linings to the working surfaces of the discs. When the pedal is released, the pads return to their original position, releasing the disc and braking stops.

In drum rear brake mechanisms, instead of calipers, wheel cylinders are used, the pistons of which open the brake shoes, pressing them with friction linings to the inner working walls of the drums. When the pedal is released, a special mechanism limits the reverse movement of the shoes, automatically maintaining the gap at a given level.

Due to the described design features, neither the front nor the rear brakes require additional adjustments.

The parking brake is actuated by a lever located between the front seats of the vehicle. The lever is connected by means of a cable drive to the shoes of the brake mechanisms of the rear wheels, which brake the car.

Master brake cylinder (GTZ) with integrated pressure control valves

GTZ is designed to create pressure of the working fluid in the hydraulic circuits of the vehicle's brake system. The cylinder is fastened with two M10 nuts on the studs of the vacuum booster servo drive.

GTZ from a body made of cast iron, inside which a cylinder hole with a diameter of 22 mm is machined, in which pistons are placed (3) And (10) drive of two circuits of the brake system. In the horizontal plane of the GTZ, four holes with internal threads are made. Two left holes with M6x1.5 thread are designed for screwing pressure control valves in the rear wheel brake circuit. Right holes (M10x1.0) are used to connect the hydraulic lines of the front brake circuits. In the upper part of the GTZ body, equipped with rubber sealing bushings are provided (5) And (12) nests for landing brake fluid reservoirs.

The rear section of the GTZ is connected to the brake mechanisms of the right front and left rear wheels (first hydraulic circuit). Front - to the mechanisms of the left front and right rear wheels (second circuit).

in the piston (3) drive of the first brake circuit, a recess is provided for fitting the tip of the pusher rod of the vacuum brake booster.

Sealing cuffs (7) And (14) serve to seal the working spaces of the corresponding sections of the GTZ. Cuff (9) prevents the penetration of hydraulic fluid from one section to another, and the cuff (2) - air entering the cylinder.

Springs (8) And (15), installed under the pistons, provide the reverse stroke of the latter. Spring (8), in addition, Serves to increase the force applied to the piston of the second section during braking.

Pins (4) And (11) restrict piston return.

When the foot brake pedal is depressed, the movement of the pusher of the servo drive of the vacuum brake booster is transmitted to the piston (3) GTZ and moves it in the cylinder. When displaced, the piston compresses the spring (8), which, together with the increased pressure of the hydraulic fluid, displaces the piston (10). Cuffs planted on the pistons (7) And (14) at the same time, they are displaced outside the inlet openings of the brake fluid reservoirs. As a result, the pressure in both hydraulic circuits of the system increases.

Note. The increase in pressure in the GTZ causes an even stronger pressing of the lips of the sealing cuffs against the cylinder walls, further increasing the tightness of the assembly.

When the pedal is released, the pistons return to their original position under the influence of the forces developed by the springs. The inlet openings communicating with the cylinder cavities open and the hydraulic fluid in the tanks freely enters the cylinders, automatically compensating for the increase in the volume of the tract associated with wear of the friction linings and / or leaks in the lines.

In case of damage to the system, causing depressurization of the first hydraulic circuit, the piston (3) will move freely, since the increase in pressure in the working cavity of this section will stop. The piston (10) under the influence of the force developed by the pusher of the vacuum booster and the spring (8), with an increased stroke of the foot brake pedal, it will ensure the correct functioning of the second hydraulic circuit. In the event of a leak in the secondary circuit, the piston (10) under the influence of the force developed by the pusher and the spring (8), overcoming the resistance of the spring (15), will move all the way with its rod to the end wall of the cylinder. In the first brake circuit, a normal increase in pressure will occur with an increased stroke of the brake pedal.

Regulator valve (s) limits the increase in pressure in the circuits of the brake mechanisms of the rear wheels, preventing premature blocking of the latter. The valve body is screwed into the outlet port of the GTZ. Seam tightness is ensured by a sealing gasket (4). An adapter is screwed into the outlet of the regulator valve body to allow connection to the brake line assembly. Brake fluid, entering the inlet of the regulator, passes into the cavity between the rubber valve (3) and the inner wall of the case. Next, the liquid enters the channel provided in the piston (7) and further, - into the brake line of the hydraulic circuit of the brake mechanism of the corresponding rear wheel. Spring (6) according to the compressive force, it is selected in such a way that at a liquid pressure of up to 20 kgf / cm² stay away from the valve. With a further increase in pressure, the spring is finally compressed and the piston is pressed against the valve, which closes the through internal hole in it. As soon as the pressure in front of the piston exceeds the pressure behind the piston by a sufficient amount, the latter shifts again, opening up fluid access to the circuit. Then the cycle repeats the necessary number of times in the process of braking the car.

Thanks to this functioning of the regulator valves, the pressure in the brake calipers of the front wheels always remains no lower than in the wheel cylinders of the rear mechanisms, which eliminates the possibility of the latter being used.

Pressure control valve located near the rear axle

On production models from December 1995, one pressure regulator valve is installed in the circuits of the brake mechanisms of the rear wheels of the car. The design of this regulator is similar to that of the regulator valves used on VW Golf cars.



The regulator adjusts the pressure in the hydraulic circuits of the brake mechanisms of the rear wheels of the car, depending on the load and position of the body.

During the routine maintenance of the vehicle, attention should be paid to the tightness of the valve-regulator connections and to check the freedom of movement of the components of its drive. In case of detection of defects, the regulator must be replaced as an assembly. Check of serviceability of functioning of the valve regulator has to be made in the conditions of service station.

Brake fluid reservoir

The two-chamber brake fluid reservoir is made of translucent plastic, which allows you to visually control the fluid level in it. The connecting branch pipes of the tank are inserted into the GTZ inlet sockets equipped with sealing bushings.

Inside the tank is divided by a partition into two independent chambers. The partition does not reach the top cover of the tank, which allows you to add liquid to both chambers at once. The separation of the circuits occurs only after the liquid level drops below the edge of the partition.

The filler neck of the tank is closed with a screw cap, in which, along with a system of channels for connecting the internal cavity with the atmosphere, the terminals of the sensor-switch of the brake fluid level and the sensor-switch itself are mounted (see chapter Onboard electrical equipment).

There are marks on the tank body for minimum and maximum liquid levels.

Vacuum brake booster

This device is designed to increase the effort applied by the driver to the foot brake pedal in order to improve braking efficiency.

The body of the vacuum booster servo drive unit consists of two parts: the body itself (5) and lids (1). The tightness of both parts of the assembly is ensured by the fit of the edges of the diaphragm (3), clamped in the housing groove. The central part of the diaphragm is hermetically sealed in the piston groove (2) and divides the internal space of the assembly into two chambers. Spring (10) provides stem return (7) together with the diaphragm to its original position.

The piston is mechanically connected to the pusher rod (7). An adjusting tip is screwed onto the end of the stem (8), with the help of which the installation gap between the GTZ piston and the pusher is adjusted (see Section Removal, condition check and installation of the control valve of the vacuum amplifier of brakes with a hose).

The tightness of the pusher seat in the assembly is ensured by a rubber gasket (6).

A valve is placed inside the assembly piston (12), against which the spherical end of the control pusher rests (15). The pusher is connected to the foot brake pedal.

At the front of the valve (12) there is a seat of a special shape, overlapped, in turn, by a valve (13). Front (vacuum) camera (4) connected by a channel (17) with piston from back (atmospheric) camera (11), located behind the piston.

Filter (16), duplicating the role of the rubber protective cover (14), prevents dust from entering the assembly when atmospheric air is sucked in.

When the foot brake pedal is depressed, the control pusher moves the first piston valve, which closes the channel connecting the vacuum chamber with the atmospheric one and opens access to the latter for outside air. The vacuum in the vacuum chamber when the engine is running is created due to the presence of a connection to the intake manifold. Due to the pressure difference on different sides of the piston, the latter shifts to the left, compressing the spring and pushing the pusher rod against the GTZ piston.

With a strong squeezing of the foot brake pedal, the piston of the vacuum booster is shifted to the left by a considerable distance. At the same time, the volume of the atmospheric chamber increases slightly and outside air is sucked into the cavity.

After releasing the pedal, the valve (4) ensures equalization of pressures on both sides of the piston, and the spring returns the diaphragm to its original state, also causing a reverse displacement of the pusher rod and, accordingly, the first GTZ piston.

The vacuum supply to the front chamber of the servo is carried out through a one-way valve included in the line connecting the block to the engine intake pipe. The valve prevents air and fuel vapors from entering the vacuum chamber of the servomotor when the engine is turned off. Thanks to this valve, the first braking with a stalled engine occurs with a working vacuum booster (after releasing the pedal, the vacuum booster ceases to function).

As follows from the above description of the principle of operation of the servo drive, it becomes possible to increase the brakes only when the engine is running, when a vacuum is maintained in the intake pipeline of the latter.

To stop a car that is being towed or coasting with a stalled engine, slightly more than the usual force should be applied to the foot brake pedal.

Spare parts for the servo drive of the vacuum brake booster are not supplied, and therefore, in case of failure, the unit must be replaced as an assembly. A description of the procedures for removing and installing the servo assembly is given in Section Check of serviceability of functioning, removal and installation of the vacuum amplifier of brakes. The landing dimensions of the block are shown in the illustration.

Front wheel brakes

The main components of a front wheel brake mechanism are a brake disc and a caliper assembly with brake pads.

The brake disc is cast from gray cast iron and is attached to the wheel hub flange with one M6x14 bolt. The inner side of the disc is protected from dirt by a special casing.

A piston is placed in the main body of the brake caliper. Two pads equipped with friction linings are inserted into the anchor bracket.

The caliper body is attached to the anchor bracket by means of one (Girgling Genuine Gears) or two bolts (license assembly mechanisms). In any case, the bolts are screwed into the holes in the ends of the guide pins. The housing can then move freely along the guide pins.

The caliper guide pins are covered with a thin layer of silicone grease and protected from dirt by rubber boots. The anchor bracket is equipped with seats for the installation of brake pads and two threaded holes for bolts, with which the entire assembly is attached to the steering knuckle of the wheel hub assembly.

Inside the caliper housing is a hydraulic cylinder with a piston. The tightness of the piston in the cylinder is ensured by a sealing collar.

The pads consist of a metal plate, on which a friction lining is glued on one side. The block is held in the saddle by a spring fixed on the plate. In parallel, the spring serves to eliminate pad vibrations.

When the foot brake pedal is depressed, hydraulic fluid acts on the caliper piston, causing the pads to be pressed against the working surface of the disc. Piston movement leads to deformation of the cuff. After releasing the pedal, the pressure of the brake fluid drops and, thanks to the spring properties of the cuff, the piston moves back, returning to the cylinder. As a result of wear of the cuffs, a moment may come when the piston, after releasing the pedal, ceases to be drawn into the cylinder. In this case, the pads will remain pressed against the brake disc and the mechanism begins to overheat.

It should be noted that as the friction linings of the pads actuate, the piston gradually moves more and more out of the cylinder. In this case, the caliper body moves along the guide pins, automatically compensating for wear.

Rear wheel brakes

The design of the drum brake mechanism of the rear wheels is shown in the illustrations.

A stamped steel brake shield is attached to the rear wheel suspension arm. Brake shoe brackets are riveted to the bottom of the shield.

The shoes have a T-section and the upper end abuts against the pushers of the wheel cylinder pistons, and the lower end rests against the bracket fixed on the brake shield. Friction linings are glued to the convex side of the shoes with special glue.

The rear shoes of the right and left brake mechanisms have the same design, while the front ones are not interchangeable due to the presence of asymmetrically located axles in the lower part for the installation of clamps for automatic adjustment of working clearances.

The shoes are interconnected by two coupling springs - upper and lower. The springs have different lengths and ensure that the ends of the shoes fit both to the pushers of the wheel cylinder pistons and to the axles.

Through the hole in the central part of the shoes, an anchor pin is threaded, fixed with a cotter pin. The pin fixes the shoe in the axial direction relative to the brake shield.

wheel cylinder) cast iron. The cylinder is mounted with a mounting flange on the shield of the brake mechanism by means of two M6x10 bolts. The flange also has a hole for the bleed valve.

Inside the cylinder are two pistons equipped with rubber sealing cuffs. An internal spring ensures that the piston pushers are constantly pressed against the brake shoes. The piston pushers have a special shape and are put on the upper edges of the shoes with a groove provided in the outer end. The wheel cylinder is protected on both sides with rubber boots to prevent dirt from entering the assembly.

The brake drum is cast iron. There are ten holes in the front wall of the drum (apart from the central). Two holes are designed for fastening the drum to the flange of the hub assembly using M8x16 bolts, four are for attaching to the wheel disk assembly, two more are auxiliary and are used when removing the drum, and the remaining two provide access to check the reverse motion of the lever of the automatic gap adjuster between the shoes and the drum.

When the foot brake pedal is depressed, the brake fluid inside the wheel cylinder causes the piston assemblies to move apart, whose pushers push the upper edges of the shoes apart, pressing the latter with friction linings against the working surface of the drum. After releasing the pedal, the shoes return to their original position due to the force developed by the coupling springs.

Excessive clearance between the shoes and the working surface of the drum leads to an increase in the free play of the foot brake pedal and, accordingly, to a delay in the operation of the brake mechanisms. In view of the foregoing, said gap should be kept to a minimum. For this purpose, the brake mechanisms are equipped with an automatic clearance adjustment device, thanks to which the controlled distance is constantly maintained within 0.4÷0.6 mm.

The expanding bar is an integral part of the automatic gap adjuster mechanism. Other components of the device include a helical-spring retainer mounted at the bottom of the front shoe, and a spring that is tensioned between the bar and a special hole in the rear shoe.

The gap between the friction linings of the shoes and the working surface of the drum depends on the gap between the teeth on the bottom of the setting lever and the teeth on the top of the latch.