The service brakes are disassembled during maintenance and troubleshooting (oiling, wear and tear of brake shoe linings, wear or damage of wheel cylinder sealing cuffs, breakage of the tension spring, etc.).
Removal and disassembly of service brake parts
Remove the wheel and hub assembly with the brake drum, clean the brake mechanism from dirt and wash it.
Using a mounting spatula as a lever, disconnect the tension spring 2.
Remove the lock clips 9, plates 8 and brake shoes 6 from the shoe axles 11.
Disconnect the brake fluid supply line from the wheel cylinder 1, unscrew the bolts securing it to the shield and remove the cylinder as an assembly.
If necessary, unscrew the bolts and remove the brake shield.
If the shoe linings or the shoes as an assembly are not to be replaced, it is not recommended to unscrew the nuts 12 and turn the axles 11 in the shield bracket, which will allow the shoes to occupy their original position relative to the brake drum during assembly, thus facilitating the adjustment of the clearances between the shoe linings and the brake drum.
When disassembling the wheel cylinder, remove the cap springs 1 (Fig. 3), the caps 2 assembled with the pistons 4.
Extract the cuffs 5, cuff holders 6 and spring 9.
After disassembly, wash the wheel cylinder parts in alcohol or brake fluid, and the parts of the mechanical part of the brake in unleaded petrol.
The washed parts should be carefully inspected and, if necessary, measured.
If the depth of the rivet heads of the lining attachment is 0.5 mm or less, the linings should be replaced.
Pistons, cuffs and cylinders with scratches or significant wear on the working surfaces should also be replaced.
The protective caps of the cylinders should be inspected with particular care, as their condition determines the durability and reliability of the wheel cylinder as a whole.
In the event of through tears and cracks, the cap must be replaced using the device (Fig. 4).
The dimensions for checking the main parts of the service brake are given in Table 1.
Table 1. Dimensions for checking the main parts of the service brake
Brake shoe support bracket:
- - nominal diameter of holes for shoe axles - 22+0.045 mm;
- - permissible diameter of holes for shoe axles - 22.10 mm
Brake shoe axle:
- - nominal outer diameter of the journal for the shoe bushing - 28-0.025 mm;
- - permissible outer diameter of the journal for the shoe bushing - 27.85 mm
Wheel cylinder:
- - nominal diameter of the piston bore - 35+0.027 mm;
- - permissible diameter of the piston bore - 35.06 mm
Wheel cylinder piston:
- - nominal outer diameter - 35.00-0.075 mm;
- - permissible outer diameter - 34.85 mm
Brake shoe assembly:
- - nominal inner diameter of the bushing - 28+0.033 mm;
- - permissible inner diameter of the bushing - 28.10 mm
Brake drum:
- - nominal inner diameter of the drum - 420+0.380 mm;
- - permissible inner diameter of the drum - 424.40 mm
In case of significant wear and the presence of annular grooves on the working surface of the brake drum, it, as well as the shoes with their linings, should be machined to one of the repair dimensions indicated in Table 2.
Table 2. Repair dimensions
Brake drum inner diameter, mm:
- Nominal - 420+0.38
- First repair - 421+0.38
- Second repair - 422+0.38
- Third repair - 423+0.38
Outer diameter of shoes by linings (2R), mm:
- Nominal - 420-0.60
- First repair - 421-0.60
- Second repair - 422-0.60
- Third repair - 423-0.60
During machining, the brake drum should be positioned with respect to the inner surfaces of the outer rings of the hub bearings, and the shoes as shown in Fig. 5.
Assemble the wheel brake in the reverse order of disassembly, observing the following mandatory requirements:
- - lubricate the wheel cylinder mirror, piston and sealing cuff with brake fluid. Apply graphite lubricant to the shoe bushings.
- - before connecting the wheel cylinders to the vehicle's hydraulic system, replace the brake fluid and clean the fluid reservoir on the brake boosters of sludge.
- - before installing the hub with the brake drum, bring the shoes inward with the adjusting eccentrics 4 (Fig. 2) and set the shoe axles so that the installation marks on the ends of the axles are directed: for the front shoe axle backwards, for the rear shoe axle forwards.
If during disassembly the position of the shoe axles on the brake shield has not been disturbed and the shoes have been placed in their places during assembly, adjust the clearances between the shoes and the brake drum only with the eccentrics 4, turning them (see Fig. 2) until the wheel is braked.
Then turn the eccentric in the opposite direction until the wheel rotates freely (the wheel should be rotated in the direction of the shoe being adjusted).
After adjustment, check the clearance through the opening in the brake drum with a 200 mm long feeler gauge at a distance of 30 mm from the edges of the lining.
The clearances should be at the toe (upper part of the shoe) 0.35 mm, at the heel (lower part of the shoe) 0.20 mm.
After replacing the brake linings or installing other shoes as an assembly, as well as when the position of the shoe axles on the shields is disturbed, the clearances should be adjusted with the eccentrics 4 and the shoe axles 11.
To do this, brake the rotation of the wheel by turning the eccentric 4.
By turning the shoe axle 11, determine the direction of its rotation in which the wheel rotates freely.
Continue to rotate the shoe axle in the same direction until the wheel is completely braked.
Divide the angle of rotation of the shoe axle from the first braked state of the wheel to the second in half and set the axle in this position.
Repeat this operation several times. Each time, the angle of rotation of the shoe axle from the first braked state of the wheel to the second will decrease.
After completing the adjustment, create a clearance of 0.20 mm at the heel of the shoe by inserting a 0.20 mm feeler gauge between the drum and the shoe heel and slightly clamping it by rotating the shoe axle.
The clearance at the toe of the shoe should then be 0.35 mm. If the clearance at the toe of the shoe is different, continue the adjustment.
After adjusting the brakes of all wheels, install the axle shafts and bleed (fill) the hydraulic system with brake fluid, having previously thoroughly cleaned the outer surfaces of the master brake cylinders, brake fluid reservoirs and bleeder valves from dirt.
It is only possible to fill the system with fluid and bleed the brakes if there is air (7.5 kgf/cm²) in the vehicle's pneumatic system.
Before filling, carefully remove dirt from the master cylinders and reservoirs.
Then, after removing the sealing tubes 1 (Fig. 6) from both cylinders and unscrewing the filler hole plugs 2, fill the reservoirs with brake fluid.
Remove air from the master cylinders through the bleeder valve 3.
Bleeding sequence for master cylinders and service brake cylinders:
Remove the rubber cap from the bleeder valve 3 (Fig. 6), put the tube from the tool kit (Fig. 6) on it, and immerse its open end in the brake fluid poured into a glass container with a capacity of at least 0.2 litres;
Unscrew the bleeder valve ½–¾ of a turn, then depress the brake pedal several times.
Depress quickly, release slowly. Repeat depressing until air bubbles stop coming out of the tube.
During bleeding, it is necessary to add fluid to the reservoirs, avoiding a "dry bottom", as this will allow air to re-enter the system.
With the pedal depressed, tightly screw in the cylinder's bleeder valve, remove the tube and put on the cap.
Similarly, bleed the wheel brake cylinders through the bleeder valve (Fig. 6) in the following sequence (Fig. 6):
- I — middle left,
- II — rear left,
- III — rear right,
- IV — middle right,
- V — front right,
- VI — front left.
After bleeding all cylinders, top up the fluid in the reservoirs to a level of 15–20 mm below the top edge of the filler neck, tightly screw in the filler hole plug, and connect the sealing tube.
When replacing the brake fluid, disassemble the wheel and master cylinders, and wash the working surfaces of the parts.
When assembling the wheel cylinders, lubricate the piston and the inner surface of the cylinders with castor oil.
