114
When checking the air-tightness the nominal pressure in the pneumatic drive should be
equal to 588 kPa (6.0 kgf/cm
2
), consumers should be working and the compressor should be
turned off.
The pressure drop in the reservoirs from the nominal level shall not exceed 49 kPa (0.5
kgf/cm
2
) during 30 min with the free controls and during 15 min after they are actuated.
When the pressure governor reacts to the compressor discharge the adsorbent of the oil-
and-moisture separator is blown out by dry air from the regeneration reservoir.
The filtering element should be replaced if necessary, when condensate appears in the
pneumatic system reservoirs.
5.5.3.2 Parameters for air drive checking:
- pressure governor:
а
) maximum disengagement pressure 870 kPa (8,7 kgf/cm
2
);
б
) minimum engagement pressure 720 kPa (7,2 kgf/cm
2
);
- four-circuit safety valve: pressure of circuits static closing is 450 kPa (4,5 kgf/cm
2
);
- trailer brakes control valve: corresponding inlet and outlet pressure of 210 kPa (2,1
kgf/cm
2
) at the control pressure of 150 kPa (1,5 kgf/cm
2
).
5.5.4 Emergency brake system
One of the circuits of the service brake system functions as emergency brake. If one of
the circuits fails the emergency system ensures the truck braking with sufficient efficiency.
5.5.5 Parking brake system
5.5.5.1 Parking brake system is used for holding the truck on the grades. The parking
brake has air drive. The brake is controlled by the brake valve with manual control by the handle
located to the right of the driver’s seat
5.5.5.2 Parking and emergency brakes air drive operation. Compressed air from the
reservoir 23 via return valve 24 (see Fig. 82) flows to the parking brake control valve 21, then to
the control line of the acceleration valve 20. As a result, the latter blows air from reservoir 23 into
the spring brake cylinders 15.
When applying the parking brake (the valve 21 handle is fixed as BRAKED) air from the
control line of the acceleration valve 20 goes into the atmosphere. The springs are loosened and
actuate the wheel brake mechanism.
The parking brake control valve is a servosystem which controls the truck deceleration
rate depending on the position of the valve handle.
In case of emergency pressure drop in the drive circuit of the parking brake, the spring
brakes 15 actuate automatically and the truck is braked.
5.5.5.3 The acceleration valve of the parking brake is shown in Fig. 95. Compressed
air from air reservoir is supplied to outlet IV. Outlet I is connected to the emergency and parking
brakes valve, outlet II is connected to the spring brakes. The control pressure from service brake
system is supplied to the outlet V. By this the air from parking brake system reservoir is supplied
into the spring brakes.
