Brake System

Page 29

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Volvo Bus Service and Maintenance Manual

 Pressure governor
52 Apply the parking brake. Check the cut-in and cut-out pressure.
 Tightness test
53 Charge the system to 700 kPa (7.0 kgf/cm2), depress the brake pedal, release the parking brake and stop the engine. Check the pressure drop.

The cut-out pressure should be 1.18-1.22 MPa (11.8-12.2 kgf/cm2). Lower the pressure in the circuit tanks by lightly braking. At the same time a mechanic can feel with his hand the air dryer drain valve. Read-off the pressure (pressure gauge 6926) when air stops flowing out of the air dryer and the pointer starts to rise.

Air dryer drain valve

The cut-in pressure should be 0.11-0.16 MPa (1.1-1.6 kgf/cm2) lower than the cut-out pressure. Note! A small amount of air hisses out of the air dryer also during charging due to the air dryer's blow-clean function.

Checking the pressure governor, brake system without pressure limitation

When checking the cut-in and the cut-out pressure, first boost the system to the cut-out pressure, 790-830 kPa (7.9-8.3 kgf/cm2). Then lower the pressure by braking a number of times and read-off when the compressor engages. The cut-in pressure should be 690-790 kPa (6.9-7.9 kgf/cm2).

Air pressure guages

Tightness test

Keep the brake pedal depressed for 2 minutes and read-off the pressure gauge. Pressure must not drop more than 40 kPa (0.4 kgf/cm2).

With a greater pressure drop, visually inspect all compressed-air lines, brake lines and hoses for leakage, scuffing or other damage. Listen for leakage in the emergency lines.

Graphic, brake lines