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