DAF LF45, LF55 Series. Manual — part 446

©

200436

2-5

Description of components

OPERATION OF BRAKE COMPONENTS

ΛΦ45/55 series

6

3

2.3 WATER BLOW-OFF VALVE

Purpose
The purpose of the water blow-off valve is to

enable any condensation in the air reservoir or air

pipes to be drained and, if necessary, to bleed the

system.

Operation
The valve is kept closed by the spring and the

reservoir pressure. By pushing the pin sideways,

the valve is lifted off the seat, allowing

condensation and compressed air to escape.

When the pin is released, the valve is closed.

Check that no other components are present

under the blow-off plug, as these could get fouled

during the blow-off process.

R600046

OPERATION OF BRAKE COMPONENTS

2-6

©

200436

Description of components

3

ΛΦ45/55 series

6

2.4 FOOT BRAKE VALVE

Purpose
The purpose of the foot brake valve is to allow

sensitive aeration and bleeding of both service

brake circuits, independently of each other.

Operation
The foot brake valve consists of two adjacent

parts (Circuit 1 and circuit 2).

If the brake pedal is depressed, a push rod will

exert pressure on the pressure plate (1). The

pressure plate will force the thrust piece (2)

downwards and close the bleed vent together

with the operating cylinder (3) and the shut-off

valve (4). If the brake pedal is depressed further,

the shut-off valve will force the control piston (5)

from its seat, causing a connection to be formed

between the supply and the outlet (brake

pressure). Due to the increase in pressure above

the control piston (5), it will be pressed down

against the spring tension. This action will

stabilise the pressure in P21-22 to the desired

value. If the brake pedal is released, the

operating piston (3) will be pushed up by the

spring tension and the bleed vent will be opened.

This will cause the pressure in P21-22 to drop

and the control piston (5) to be pushed up by the

spring tension, closing the connection between

the supply and outlet.

R600588

R600570

P21

P22

P11

P12

2

3

5

1

©

200436

2-7

Description of components

OPERATION OF BRAKE COMPONENTS

ΛΦ45/55 series

6

3

2.5 RELAY VALVE

Purpose
The purpose of the relay valve is to allow fast

aeration and bleeding of the spring brake

cylinders and brake cylinders, shortening the

brake reaction/release time.

Note:

The hysteresis of the relay valve, which is used

for the parking brake, is greater and therefore is

not suitable for use in the service brake.

The air reservoir is connected to point 1. When

connecting point 4 is pressureless, inlet 5 is

closed and outlet 6 opened. The brake chambers

connected to point 2 have now been bled.

When compressed air passes through

connecting point 4 into chamber "a" above the

piston (7), the piston is forced downwards. Outlet

6 is closed and inlet 5 opened. The compressed

air now passes from the air reservoir to the brake

chambers.

A state of equilibrium is achieved when the

pressures on both sides of the piston (7) are

equal. Then, both the outlet and the inlet are

closed.

The rubber flap over opening 3 prevents dirt from

entering, whilst providing a large opening for air

to be bled.

When the pressure in connecting point 4 and

consequently in chamber "a" drops, the piston (7)

is forced upwards. Inlet 5 is closed and outlet 6

opened and as a consequence the brake

chambers are bled through bleed opening 3.

R600490

2

1

6

4

A

5

3

7

R600491

2

1

6

4

A

5

3

7

OPERATION OF BRAKE COMPONENTS

2-8

©

200436

Description of components

3

ΛΦ45/55 series

6

2.6 EMPTY/LOAD RELAY VALVE

Purpose
The purpose of this valve is to adjust the braking

pressure to the front axle depending on the

output pressure from the load sensing valve of

the rear axle.

R600049