Brake System

Page 28

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Volvo Bus Service and Maintenance Manual

 Brake system
50 Check/drain condensation water from the air tanks. Automatic drain valves are checked by pushing in the pin in the lower part of the valve. Clean the filter at the outlet on the wet tank (if fitted). Drain the air tank of pressure before dismantling the filter.
 
 Air pressure build-up
51 Apply the parking brake. Run the engine at about 1000 r/min and check the air pressure build-up.
 At the same time check the alternating function of the air dryer (twin-tower type).
 

Brake system

Water in the air tanks indicates a faulty air dryer (see Step 60) or automatic drain valves (if fitted). Damp air feels cold, dry air warm.

Change the compressor air filter when replacing the air cleaner filter insert. This applies to compressors with separate filter.

Normal charging refers to a rise in pressure from 200 to 400 kPa (2-4 kgf/cm2), in 1 to 2 minutes when both circuits are charged simultaneously, or in 30 seconds to 1 minute when one circuit charges at an engine speed of 1000 r/min. The charging capacity gives you some idea of the condition of the compressor.

Read-off the pressure when the warning lamp for the service brakes goes out. Correct pressure is 460-520 kPa (4.6-5.2 kgf/cm2).

Compressed-air brake system, constant-pressure type

Air dryer, checking tower alternating function

With the feed switch on, the solenoid valve should alternate between the towers at intervals of one minute. This is indicated by a clicking sound and a light puff can be felt at the discharge pipe under the air dryer. If no tower change takes place after more than one minute, the solenoid valve alternating function is faulty, which means that it must be replaced by a new one.

Air dryer, checking tower alternating function

Pressure governor, checking cut-in and cut-out pressure (52)

Lower the wet tank pressure to approx. 1.0 MPa (10 kgf/ cm2). Start the engine and let it run at low idle. Read-off the pressure (pressure gauge 6926) when the pointer stops rising and the air dryer drain valve opens (denoted by a powerful hissing sound).

Pressure governor,
    checking cut-in and cut-out pressure