Attention! Before proceeding with the procedures, carefully read the warning text at the beginning of the Section System design, description of individual units and mechanisms.
New mounting nuts and sealing ring will be required for assembly.
Removal
Models without ABS
1. Thoroughly wipe the surface of the master cylinder reservoir and remove the cover. Remove the filter and pump out the hydraulic fluid from the reservoir.
Note: Never suck out brake fluid by mouth - use a syringe or rubber bulb! Alternatively, open the bleed valve of any of the brake mechanisms, connect a drain hose to it and empty the reservoir by pumping the fluid into a drain container by pressing the foot brake pedal.
2. Release the reservoir retaining tab and remove the reservoir from its bearing bushings on the master cylinder.
3. Wipe the cylinder wall surfaces around the brake line unions. Place a rag under the unions to collect any spilled fluid. Remembering the order in which the tubes are connected, loosen the union nuts and disconnect the lines. Immediately plug the open ends of the tubes and unions to minimize brake fluid loss and prevent dirt from getting into the system. Collect any spilled fluid with a rag.
4. Loosen the nuts securing the brake master cylinder to the brake booster servo unit and remove the washers.
Note: During assembly, nuts must be replaced without fail.
5. Remove the brake master cylinder from the vacuum booster servo unit together with the sealing ring.
Note: The sealing ring must be replaced during assembly.
6. Due to the lack of replacement internal components on sale, major repairs of a failed brake master cylinder are not possible. The defective assembly should be replaced with a new one. The only components that need to be replaced are the rubber bushings of the reservoir mounting supports on the brake master cylinder. Both bushings are simply tightly seated in their sockets.
ABS equipped models
Note: On models equipped with ABS, before disconnecting the hydraulic brake lines, be sure to disconnect the battery. Connect the battery only after all hydraulic lines of the brake system have been connected. Failure to do so may result in air entering the hydraulic modulator, which is removed during normal bleeding of the system (see section text) is not possible and requires the use of a special electronic tester (VAG 1552).
1. Disconnect the negative battery cable, then remove the cap from the brake master cylinder reservoir. Pump out the hydraulic fluid from the reservoir.
Note: Never suck out brake fluid with your mouth - use a syringe or rubber bulb!
2. Remove the retaining pin of the locking tongue of the retainer and carefully remove the reservoir from the master cylinder body, releasing it from the rubber sealing bushings.
3. Wipe the cylinder wall surfaces around the brake line unions. Place a rag under the unions to collect any spilled fluid. Remembering the order in which the tubes are connected, loosen the union nuts and disconnect the lines. Immediately plug the open ends of the tubes and unions to minimize brake fluid loss and prevent dirt from getting into the system. Collect any spilled fluid with a rag.
4. Loosen the nuts securing the brake master cylinder to the brake booster servo unit and remove the washers.
Note: During assembly, nuts must be replaced without fail.
5. Release the hydraulic modulator support bracket from the servo unit, then remove the master cylinder with the sealing ring.
Note: The sealing ring must be replaced during assembly.
6. Due to the lack of replacement internal components on sale, major repairs of a failed brake master cylinder are not possible. The defective assembly should be replaced with a new one. The only components that need to be replaced are the rubber bushings of the reservoir mounting supports on the brake master cylinder. Both bushings are simply tightly seated in their sockets.
Major repairs
1. Brake fluid leaks developing between the master cylinder flange and the brake booster servo housing, or an increase in the foot brake pedal travel indicate the need for a major overhaul of the cylinder assembly. It is recommended that the repair be carried out in a service station workshop.
2. Remove the master cylinder.
3. Release the brake fluid reservoir from the brake master cylinder receiving sockets.
4. Remove the rubber bushings from the sockets.
5. Remove the piston stroke limiting pins from the cylinder inlet ports - just turn the assembly upside down and shake it.
6. Turn the cylinder vertically and, striking the seat flange against a wooden block, knock out first one piston, then the second.
7. Carefully inspect the mating surface of the cylinder body for cavities and mechanical damage. Minor defects can be removed by grinding with a fine-grained polishing paste. After grinding, wipe the treated surface with methyl alcohol. In case of more serious damage, the cylinder should be replaced.
8. Wash all removed components in clean brake fluid or methyl alcohol.
Note. Never use gasoline or other petroleum-based solvents to wash the GTC components, as removing the contact will destroy the rubber elements. After washing, carefully examine the condition of the parts, paying special attention to the elasticity of the piston cuffs (you can use a screwdriver to remove old cuffs from the pistons; using a screwdriver to install new seals may result in irreversible failure).
9. Also check the condition of the rubber bushings removed from the sockets for the brake fluid reservoir connecting pipes.
10. After replacing worn or failed components, you can begin assembling the device (see below).
11. Wet the cylinder mirror with clean brake fluid.
12. Install sealing cuffs on the pistons and washers for installing springs on their rods.
13. Place the return spring on the shorter piston rod (with the smaller diameter end [shorter pitch coils?] facing forward). Insert the piston and spring assembly into the cylinder (spring forward). Using a punch placed against the bottom of the piston, compress its spring by approximately 0.5 cm and, holding the assembly in this position, insert the corresponding piston stroke limiting pin into the inlet port.
14. Proceeding in a manner similar to that described in the previous paragraph, place the return spring on the second piston rod. Insert the piston and spring assembly into the cylinder (spring forward). Insert the drift into the recess in the end of the piston and, overcoming the resistance of the spring, push the piston approximately 2 cm below the mating surfaces of the mounting flange. Holding the piston in this position, insert the thrust pin into the second inlet of the cylinder.
15. By pressing the punch several times, check the freedom of movement of the pistons in the cylinder.
16. Remove the drift and firmly seat the rubber bushings into the holes for the thrust pin.
17. Before installing the master cylinder assembly, check the amount of protrusion of the adjusting tip of the vacuum booster servo push rod and the depth of the piston seat in the cylinder (distance from the mating surface of the flange to the bottom of the recess in the piston). The difference between the results of the first and second measurements should be 0.3÷0.6 mm. The presence of such a gap is a mandatory condition for the proper functioning of the master cylinder and vacuum booster. The gap is adjusted by rotating the tip at the end of the pusher rod. Note: In a service station workshop, the installation gap of the pusher is determined using a special device.
18. Now you can proceed to installing the assembly.
Installation
1. Carefully clean the mating surfaces of the master cylinder and the vacuum booster unit. Install a new sealing ring into the groove on the rear side of the cylinder body.
2. Position the master cylinder on the vacuum booster block. Make sure that the servo push rod enters the center of the cylinder piston. Install the washers, screw on new mounting nuts and tighten them to the required torque.
3. Wipe the union assemblies of the brake pipes and connect the latter to their original places on the master cylinder. Tighten the union union nuts securely.
4. Lightly lubricate the rubber sealing bushings on the top of the master cylinder with fresh brake fluid and fit the hydraulic fluid reservoir tightly into them. Secure the reservoir to the cylinder by snapping the locking tab of the retainer.
5. Fill the brake master cylinder reservoir with fresh hydraulic fluid (up to the MAX mark) and perform a complete bleeding of the hydraulic system (see section Bleeding the hydraulic system). Replace the reservoir cap. Before operating the vehicle, check that the brake system is functioning properly.
ABS equipped models
1. Carefully clean the mating surfaces of the master cylinder and the vacuum booster unit. Install a new sealing ring into the groove on the rear side of the cylinder body.
2. Move the hydraulic modulator away from the vacuum booster servo unit and install the master cylinder in its regular place. Make sure that the servo pusher rod enters the central part of the cylinder piston. Return the modulator to its place, placing its bracket on the vacuum booster unit studs, then install the washers, screw on new fastening nuts and tighten them with the required force.
3. Lightly lubricate the rubber sealing bushings at the top of the master cylinder with fresh brake fluid and fit the reservoir tightly into them. Secure the reservoir to the cylinder by installing the locking pin.
4. Before connecting the brake lines, bleed the air from the master cylinder. To do this, pour fresh hydraulic fluid into the master cylinder and place a drain container under the nipples. Ask an assistant to press the foot brake pedal several times at 5-second intervals. When the fluid free of air bubbles starts to be pushed out of the cylinder, you can stop bleeding. Pour fluid into the brake pipes as well. Ask an assistant to press the pedal and fix it in the lower position and connect the brake pipes to the cylinder. Tighten the union nuts securely. After connecting both pipes, you can release the pedal. Remove the drain container and wash off the splashes with clean cold water.
5. Fill the brake master cylinder reservoir with fresh hydraulic fluid (up to the MAX mark) and perform a complete bleeding of the hydraulic system (see section Bleeding the hydraulic system). Replace the reservoir cap. Before operating the vehicle, check that the brake system is functioning properly.
(Text copied from this resource: skodabook.ru)