Trailer braking Load Sending


The tractor trailer brake mechanism comprises a hydraulic unit with two priority blocks. It is located on the right-hand hydraulic cover plate of the rear axle housing. The priority blocks are screwed together.
The inner unit (7) ensures priority to the 17 or 21 bar system. The outer unit (10) shares priority between the steering and trailer braking. Flow is then directed to the auxiliary spool valves and linkage spool valve.
For the 40 kph version with trailer braking, the tractors are fitted with a hydraulic unit with two priority blocks. A trailer brake valve is integral with the outer priority block (10). It is supplied by the control pressure from the brake master cylinders when the brake pedals are locked together.
For the version with high-pressure braking with or without a trailer brake, tractors are systematically
fitted with a hydraulic unit with two priority blocks. These priority blocks can be distinguished from the priority blocks on the 40 kph version with trailer braking by a higher number of hydraulic ports. These additional hydraulic ports (M) and (N) are used by the high-pressure braking unit. A tractor trailer brake valve is integral with the outer priority block (10). It is supplied by the control pressure from the block/valve assembly when the brake pedals are locked together.



Identification of 110 l/min Load Sensing ports

Parts list 
(A) Supply to steering spool valve (Orbitrol)
(B) High-pressure supply from variable displacement pump
(C) LS pilot flow to the variable displacement pump regulator
(D) LS pilot flow from the steering spool valve (Orbitrol)
(E) To trailer brake connector
(F) Pressure to linkage valve and auxiliary spool valves
(G) LS pilot flow from linkage valve and auxiliary spool valves
(I) 17 or 21 bar outlet (J) Plugs
(K) Pilot flow from brake master cylinders (40 kph tractors) or block/valve assembly (tractors fitted with high-pressure braking)
(M) High pressure internal channel to the braking unit (tractors fitted with high-pressure braking)


(N) LS pilot flow from the braking unit which is directed to the variable displacement pump regulator via a channel inside the priority block
(10) (tractors fitted with high-pressure braking)
(O) Vent port (trailer brake line)
(P. dir) Steering priority spool
(BP) Low-pressure spool
(HP) High-pressure spool
A. Hydraulic unit with two priority blocks for 40 kph tractors
B. Hydraulic unit with two priority blocks for tractors fitted with high-pressure braking

Parts list 
(A) Supply to steering spool valve (Orbitrol)
(C) LS pilot flow to the variable displacement pump regulator
(D) LS pilot flow from the steering spool valve (Orbitrol)
(E) To trailer brake connector
(F) Pressure to linkage valve and auxiliary spool valves
(G) LS pilot flow from linkage valve and auxiliary spool valves
(I) 17 or 21 bar outlet (K) Pilot flow from brake master cylinders (40 kph tractors) or block/valve assembly (tractors fitted with high-pressure braking).
(M) High pressure pipe to the braking unit (tractors fitted with high-pressure braking) (N) LS signal pipe from the braking unit (tractors fitted with high-pressure braking) (O) Vent port (trailer brake line)
B - Hydraulic unit with two priority blocks for tractors fitted with high-pressure braking For tractors fitted with high-pressure braking without trailer braking, the hydraulic port (K) is blanked off with a threaded plug

Spool operation


A - Priority block (10) GPA20, GPA30 and GPA40 rear axles for 40 kph tractors 
B - Priority block (10) GPA20 and GPA40 rear axles for tractors fitted with high-pressure braking 

Tractors fitted with high-pressure braking have an outer block (10) which has two additional hydraulic ports (M) (pressure) and (N) (LS signal).

Steering priority spool (2) 


The spool moves to the left under the action of spring (5). The flow from the channel (B) is directed as a priority to the steering spool valve (Orbitrol) via the channel (A). With the steering in a neutral position, there is no flow in this channel. The increasing pressure forces the spool to move to the right and compress spring (5) thus allowing the flow to pass into channel J. During operation, a pilot flow pressure from the steering spool valve (Orbitrol) arrives through channel (D) and pushes the spool back to the left, thus allowing the flow rate required by the steering to enter into channel (A). The remaining flow is directed into channel J.

Trailer brake spool (6) 
When the trailer brake is not activated, the pressure in channel K is zero and the spool allows communication between channels L and R. As soon as the tractor brake is activated, the pressure existing in channel K moves piston (7), which pushes the spool to the left, allowing the flow to pass between channels J and L. The increasing pressure in channel L acts on the end of the spool (6) and compresses the spring (8) to create a balance allowing the pressure in channel L to be maintained depending on the pressure in channel K. This pressure is maintained by linking channel L either to channel J or port R. As with the "no trailer braking" priority block, a flow rate of 0.5 l/min flows through channel A towards channel D (dynamic steering) via a hole and a restrictor in the spool (2).

Parts list 
(P1) High pressure hydraulic line
(P2) Provision for high pressure connector
(R) Return below oil level
(LS) Signal to the variable displacement pump regulator via the hydraulic unit with two priority blocks
A - Hydraulic unit with two priority blocks for 40 kph tractors
B - Hydraulic unit with two priority blocks for tractors fitted with high-pressure braking