Servicing front disc brakes

Note. You will need the following special tools to perform the procedures below: Dial meter (to check the amount of disc runout), vernier caliper (for measuring disc thickness) and steelyard (for measuring the drag force of the brakes).

Checking the condition and replacing the brake pads

1. Jack up the front of the car and place it on jack stands. Remove the front wheels.

2. Through the viewing window in the caliper body (see accompanying illustration) evaluate the residual thickness of the friction linings of the brake pads. If the lining thickness of any of the pads is less than the allowable (see tables of sizes and adjustments at the end of the guide), or if the pads are contaminated with grease or brake fluid, replace all four pads on the brakes on both wheels.

Note. If the pads are dirty, you must also find and eliminate the cause of the leaks.

3. Using a syringe, pump out approximately two-thirds of the nominal amount of brake fluid from the GTZ reservoir.

Note. The used liquid cannot be reused.

4. The liquid is removed in order to prevent it from flowing over the edge of the reservoir when the caliper pistons are sunk to replace the pads.

Attention! Do not remove all fluid from the reservoir, as this will require bleeding the hydraulic system.

5. Loosen the bottom guide pin bolt and rotate the caliper body up on the top pin (see accompanying illustration), opening access to the brake pads, shims and holders in the anchor bracket (guide pads) caliper.



6. Remember the installation position of the components and remove the holders, blocks and shims from the anchor bracket.

7. Check shims and holders for signs of wear and damage. Replace defective components.

8. Install the shims on the shoes, then install the anchor bracket in their seating positions (with gaskets) and holders. Lightly lubricate the holders with silicone grease.

9. Push the piston perpendicularly into the caliper cylinder. Use a clamp if necessary.

10. Make sure that the caliper slides freely on the guide pins, then lower it onto the blocks.

11. Install the lower guide pin and tighten its bolt to specification.

12. After completing the replacement of the pads of both brake mechanisms, depress the brake pedal several times in order to shrink the pads on the discs. If necessary, add brake fluid to the reservoir to the correct level.


13. Install the front wheels and lower the vehicle to the ground.

14. Carry out a road test of the car, checking the proper functioning of the brakes.

Caliper overhaul

1. Jack up the front of the car and place it on jack stands. Remove the front wheels. Front disc brake caliper components are shown in the accompanying illustration.



2. Give a union bolt and disconnect a brake hose from a support. Cap the end of the hose to prevent dirt from entering the hydraulic system and to minimize brake fluid loss.

3. Give bolts of directing fingers of a support and remove the case of the last from assembly of an anchor bracket.

4. Press a piece of wood against the anchors of the caliper body pads and carefully depress the piston from the caliper cylinder while applying low pressure air to the hydraulic hose fitting hole on the caliper body.

Note. Excessive air pressure can cause the piston to extrude at high speed, resulting in personal injury or piston damage.

5. Remove the boot from the piston and remove the piston cuff from the cylinder.

Note. To remove the cuff, use a blunt tool, preferably made of plastic or wood.

6. Remove the guide pins and shoe components from the anchor bracket.

7. Wash the caliper cylinder and piston with clean brake fluid.

8. Check the cylinder bore for cavities, corrosion, signs of wear and damage. Replace the caliper body if necessary.

Note. Minor defects can be removed with fine sandpaper.

9. Check the piston for cavities, corrosion, signs of wear and damage. Replace if necessary.

Attention! Do not attempt to polish the piston with fine sandpaper as this will damage the piston lining.

10. Check the condition of the shoe components, guide pins, and anchor bail. Replace defective parts.

11. Discard all rubber components removed from the caliper assembly.

12. Lubricate the new piston seal and boot with rubber lubricant and install the seal in the cylinder groove. Follow the correct fit of the cuff in the groove - it should not be twisted.

13. Put the boot on the piston and lubricate the latter with clean brake fluid.

14. Fully sink the piston in the caliper cylinder and secure the boot in the groove on the edge of the cylinder.

15. Lubricate the guide pins and the inside of their protective boots with rubber lubricant and install the pin assemblies with boots into the anchor bracket.


16. Install shoe components into anchor bracket.



17. Hook the steelyard to the wheel stud and measure the resistance force of the hub to turning (see accompanying illustration). Write down the result of the measurement. If necessary, check the wheel bearing preload adjustment.



18. Using a dial gauge mounted on the front suspension component with a plunger pressed against the end surface of the brake disc, measure the amount of disc runout (see accompanying illustration). If the measurement result is out of range (see tables of sizes and adjustments at the end of the guide), grind the disc, or replace it.

19. Using a vernier caliper, measure the thickness of the brake disc at several points along the generatrix. If necessary, grind the disc or replace it.

20. Put the caliper body on the anchor bracket assembly, screw in and tighten the guide pin bolts with the required force. Connect a brake hose to a support and tighten a union bolt with the demanded effort.

21. Bleed the hydraulic brake system.

22. Depress the brake pedal for five seconds.

23. Release the pedal and rotate the front wheel hub ten turns.

24. Using the steelyard, measure the resistance force of the hub to turning.

25. Subtract the result of the first measurement from the result of the second in order to determine the drag force of the brake. The maximum allowable value is 7.2 kG, if it is exceeded, look for jammed components and make the necessary repairs.

Removal and installation of a brake disk

Removing

1. Remove the front wheel hub assembly.

2. Give fixing bolts and separate a disk from a nave.

Installation

Installation is in the reverse order.

1. Tighten bolts of fastening of a disk to a nave with the demanded effort.

2. Adjust wheel bearing preload.