The pneumatic brake booster (Fig. 1) is designed to convert compressed air energy into the required pressure of the working fluid in the hydraulic part of the brake drive.
To increase brake reliability, Ural vehicles are equipped with two brake boosters, installed on the outer side of the left frame side member under the cab.
The front brake booster actuates the service brakes of the front and middle axle wheels, the rear one actuates the service brakes of the rear axle wheels.
When the brake pedal is depressed, a valve opens in the brake valve, and air is supplied through the pipeline under the pistons of the pneumatic booster.
Under air pressure, the rod with the pistons moves and, through the pusher, acts on the master brake cylinder piston, which displaces the fluid into the brake line.
When releasing the brakes, air from the pneumatic booster exits to the atmosphere through the brake valve.
The pistons of the master brake cylinder and the pneumatic booster return to their initial position under the action of springs.
Remove the brake boosters from the vehicle and disassemble them only for maintenance and troubleshooting (replacement of worn seals, etc.).
When removing the brake boosters from the vehicle, disconnect the pipelines of the brake and sealing systems from them and the master brake cylinders, as well as the wires from the sensors of the «Brake» warning lamp.
Unscrew the mounting bolts to the frame side member and remove the brake boosters.
Disassembly of the brake boosters
Drain the brake fluid, unscrew and remove the reservoir 4, unscrew the nuts, remove the tie rods 22 and the rear pneumatic cylinder 19.
Remove the hose, the pistons assembled with the rod 23, the spacer 16 and the pusher 8.
Unlock and unscrew the bolts 29, remove the lock plates 30, the front brake booster cylinder 9 assembled with the cover 31 and the gasket.
Remove the retaining ring 27, the washer 28 and extract the piston 6 assembled with the outer seal 7. Extract the ring 33, the inner sealing seal 34 and the spring 36 assembled with the check valve 38.
It is not recommended to unscrew the plug 1 during disassembly.
Unscrew the nuts 21 on the rod, remove the pistons 12 assembled, the spacer 16 assembled with the seal 24 and the sealing rings 17.
Drive out the pin 26 from the rod and separate the rod 23 from the pusher 8.
Wash the master brake cylinder parts in alcohol or brake fluid, and the pneumatic booster parts in kerosene or petrol.
The data for checking the main dimensions of the pneumatic booster and master brake cylinder parts are given in the table.
Table 1.
Front and rear cylinders of the pneumatic booster. Inner diameter:
- - nominal size - 150+0.230 mm;
- - permissible size - 150.50 mm
Master brake cylinder. Inner diameter:
- - nominal size - 40+0.030 mm;
- - permissible size - 40.07 mm
Master brake cylinder piston. Outer diameter:
- - nominal size - 40-0.025 mm;
- - permissible size - 39.90 mm
If the seals in the spacer, as well as the seals of the master brake cylinder piston and the pneumatic booster, are worn or mechanically damaged, replace them. The cylinder mirror must have no corrosion or scratches.
The master brake cylinder must have no chips or cracks, and the pneumatic booster cylinders must have no dents.
Assemble the pneumatic booster in the reverse order of disassembly, while observing the following mandatory requirements:
- - apply CIATIM-201 lubricant to the working surface of the pneumatic cylinders 9, 19 (see Fig. 1), the rod 23, seals 24 and 13. Impregnate the felt rings 11 of the pistons with spindle oil;
- - assemble the pneumatic booster pistons in a ring with a diameter of 150+0.3 mm;
- - when assembling the rod 23 with pistons 12, spacer 16 and pusher 8, maintain the dimensions shown in Fig. 1;
- - install a rubber sealing ring 25 between the seal support ring 15 and the piston 12;
- - install the gasket and bolts 29 for fastening the master brake cylinder to the front cylinder of the pneumatic booster, as well as nuts 21 for fastening the pistons on the rod, using sealing grease;
- - position the spacer relative to the front cylinder so that the fittings connecting the outlet holes are aligned;
- - position the rear cover 20 relative to the front cover 31 so that the surfaces with the holes «Б» are in the same plane.
The assembled pneumatic booster must be checked for tightness with an air pressure of 4.0–4.5 kgf/cm².
The master brake cylinder must then generate a fluid pressure of 100–112 kgf/cm².
Fluid and air leaks are not allowed.
Install the brake boosters on the vehicle, connect the pipelines, and bleed the hydraulic system.
