Twin flow load sensing trailer braking

The tractors may be equipped with a trailer braking unit comprising:
- a valve unit (41) (spool valve) fitted to the balance block of the right-hand hydraulic cover
- a pipe linking the spool valve to a connector (47) located at the rear of the tractor
- a pilot flow pipe connected to the master cylinders.
The valve receives its priority supply from the high pressure circuit. The oil not used for braking the trailer is directed to the spool valves (auxiliary and lift control spool valves). The valve regulates the flow rate and pressure to the trailer brakes. It is controlled by the tractor braking circuit pressure, generated by the master cylinders, so as to obtain progressive tractor / trailer braking that is proportional to the effort applied to the pedal. The tractor trailer brake only operates if the two brake pedals are latched.

Identification of ports 
B : Supply to the trailer brake connector
N : Continuity to spool valves (auxiliary and lift control spool valves )
P :Pressure
R : Return to housing
Y : Pilot flow from tractor braking unit

Description of the braking valve 

 Flow control valve (1) ensures control of the Qx flowrate and adjusts the hydraulic flow, transmitting the pressure to the tractor trailer brakes.

Control spool assembly (2) ensures the control of the flow control valve and adjusts the pressure in the tractor trailer brakes.

Non-return valve (3) stops the return of oil from brake pipe B to port N.

Pressure relief valve (4) with loaded spring (8), limits the braking pressure.

Pilot flow housing (5) with piston (6) and bleed screw (7), controls the trailer brake valve via the tractor braking system.

Removing and install the braking valve

If the removal of the valve does not concern its replacement, do not disconnect the hose (B) from the master cylinders, in order to avoid draining the system. Immobilise the tractor. Take off the wheel concerned. Position an axle stand.

Disconnect and block off:

- the hose pipe (B) of pilot flow housing (D)
- the feed hose (A) from the auxiliary spool valve and the pipe (C) from the tractor trailer brake connector.
Take out the screws (19) and remove the valve (18).

Clean the mating faces of the hydraulic cover and the valve.
Replace the O'rings (1) (2).

Reinstall the valve, place and tighten the screws (19) to a torque of 21 - 24 Nm. Reconnect:
- the pipe (C) of the trailer brake connector and the feed hose (A) from the auxiliary spool valve
- the hose pipe (B) of the pilot flow housing.
If necessary, bleed the main brake and the trailer brake. Check the pressure in the trailer brake system. Reinstall the wheel. Remove the axle stand. Tighten the wheel nuts to a torque of 400 - 450 Nm.

Operation, trailer brakes released and partial braking

Trailer brakes released 
The control channel Y is not under pressure (no action on brake pedals). Port B (feed to the trailer brake connector) is linked to return channel R via the drilled hole f of the control spool (2). Flow QP from the pump flows via port P to the flow control valve (1). Flow QP - QX is directed towards port N (continuity towards auxiliary spool valve and lift control valve). Partial flow QX (approximately 0.6 l/min) from port P is directed towards the diaphragm (11), the restrictor (9), the drilled hole (10) and the control spool (2) to join channel R. Consequently, the pressure drop produced by the restrictor (9) maintains the flow control valve (1) in open position a, the position in which it does not assure a regulating function.

Partial trailer braking

Start of the braking process 
The piston (6) of the pilot flow housing (5) is pressure fed by the tractor braking system (action on the brake pedals). The pressure arrives via port Y, the control spool assembly (2) is then pushed to the left, obstructing the drilled hole f and cutting off the communication of port B (towards the trailer brake connector) with the return channel R, then the link with the drilled hole (10). The control spool (2) moves from position c to position e. The QX control flow is cut and the flow control valve (1) moves to position b (regulating position). A constant flow QK flows from port P to port B (towards the trailer brake connector) via the diaphragm (11), the drilled hole (12), the channel (13) and the non-return valve (3). The diaphragm (11) has a surface area calculated in relation to the constant flow rate QK. The residual flow QR flows via the flow control valve (1) and is directed to port N (continuity with the auxiliary spool valve and the lift control valve). The channel of port B (towards the trailer brake connector) is placed under pressure and acts upon the surface (14) of the control spool (2) against the pressure on piston (6) by the tractor braking system.

Partial and maximum trailer braking

Partial trailer braking 
The pressure Pb in the trailer braking line (pressure acting on the active surface (14) of control spool (2)) is in balance with pressure Py (coming from the tractor braking system) which acts upon piston (6) of the pilot flow housing (5). The channel from port B (to the trailer brake connector) is cut off from the return port R. Consequently, the oil is trapped in the trailer braking unit. When the pressures are balanced, the control spool (2) is in position d. The flow control valve (1) then moves to position a, an open position in which it does not ensure a regulating function. As when the trailer brakes are released, the flow Qp - Qx is directed to port N (continuity with the auxiliary spool valve and the lift control valve) and the control flow Qx is directed to return channel R by the control spool (2).

Maximum trailer braking 

Limited braking pressure
The flow control valve (1) and the control spool (2) are in the same switching positions (a and d) as for partial braking. The flow rates Qp and Qx are the same as for partial braking. The maximum admissible braking pressure is reached (Pb = 150 bar). An increase in the trailer braking pressure cannot take place, even if the tractor braking pressure continues to increase. The pressure relief valve (4) is then pushed to the left. The springs (8) (15) pre-loaded with the maximum admissible braking pressure (Pb = 150 bar) for the trailer, are compressed. 

If, due to external conditions, the trailer braking pressure Pb continues to rise, the control spool (2) momentarily opens the communication f between port B (supply to the trailer brake connector) and the return channel R. In all operating positions of the trailer braking valve, the auxiliary spool valve and the lift control valve are used when required by port N without causing significant effect on the trailer brake system, which has priority on the high flowrate hydraulic circuit.

Version without trailer braking 

- For this tractor version, the braking valve (spool valve) is replaced by a cover plate (3).

- As with the trailer braking version, the oil tightness between components is ensured by O’rings (1) (2).

- A test connector (4) also allows to check the high pressure of the pump.