High-pressure braking � block and valve assembly

The block/valve assembly (13) is fitted to tractors equipped with a GTA1540 or GTA2520 transmission:

- as standard on 50 kph tractors

- as an option on 40 kph tractors

It is fixed onto a support (S), which is fixed to the front right-hand side of the cab. The block/valve assembly (13) is hydraulically linked to the high-pressure braking unit (see high-pressure braking unit). It manages the high pressure that it receives from the high-pressure braking unit (1) via the brake pedals (P) and the two sets of straight bearings (r). 

The output pressure is then directed to the main brake mechanism (right and/or left-hand brake) located inside the GPA20 or GPA40 rear axles housing and inside the trumpet housings. The brake pedals may be uncoupled or coupledtogether using the latch.

Identification of ports and main components

A. Block/valve assembly without trailer braking 

B. Block/valve assembly with trailer braking 

C. Hydraulic unit with two priority blocks and closed centre 

Identification of ports

The block/valve assembly (13) is one of the pressurised control components of the tractor's main brakes. It is identical whether the tractor is fitted with a trailer brake mechanism or not. If the tractor is not fitted with this mechanism, a threaded plug blanks off the tapped port designed for the hydraulic line (FR) (trailer brake) to the block/valve assembly. The block/valve assembly is identified by its specifications, which are written on an adhesive plate (1). The block/valve assembly has the following five hydraulic lines:

(F1) Pressure delivered to the main left-hand brake

(F2) Pressure delivered to the main right-hand brake

(FR) Control pressure to the trailer brake valve

(P) High pressure from the high-pressure braking unit

(T) Return

Legend 

High pressure - red

Braking pressure - yellow

Return - green

Control - brown

Identification of the main components

These hydraulic ports are identified by the letters and numbers engraved on the body of the block/valve assembly.

The block/valve assembly (13) comprises the following main components:

(b) Spool spring and plug

(c) Block/valve assembly body

(c1) Valve

(p) Guard

(pp) Main piston and head

(t1) Spool

(t2) Spool

The diagram of the block/valve assembly (13) provides a symbolic representation of the hydraulic layout of the following main components:

- the valve (c1), which is systematically integral with the block/valve assembly whether the tractor is fitted with a trailer brake mechanism or not

- the main spool (pp), which receives the high pressure from the high-pressure braking unit

- the spool (t1) for the hydraulic line (F1)

- the spool (t2) for the hydraulic line (F2)

Operation

 The block/valve assembly's main piston (pp) and spools (t1) and (t2) are controlled:

- either in mode 1: pedals uncoupled 

- or in mode 2: pedals coupled 

Legend 

High pressure - red

Braking pressure - yellow

Return - green

Control - brown

Mode 1: pedals uncoupled 

When just one brake pedal is pressed, the main piston (pp) and the spool (t1) or (t2) (depending on which pedal is used) act simultaneously. The oil under high pressure from the high-pressure braking unit and the port (P) on the block/valve assembly passes via the main piston (pp) and is then directed to the spool (t1) or (t2). The high pressure is then directed to the relevant brake piston on the centre housing via the hydraulic line (F1) or (F2) (depending on which brake pedal is used) according to the force applied to the brake pedal. Figures shows line F1 under hydraulic activity. In mode 1, the valve (c1) simultaneously blocks the passage of oil to the hydraulic lines (F1) or (F2) (depending on which brake pedal is used) and (FR) (trailer brake, if fitted).

Mode 2: pedals coupled 

When both brake pedals are pressed, the main piston (pp) and the two spools (t1) and (t2) act simultaneously. The oil under high pressure from the high-pressure braking unit and the port (P) on the block/valve assembly passes via the main piston (pp) and is then directed to the spools (t1) and (t2). Like in mode 1, when the brake pedals are uncoupled, the high pressure is then directed to the brake pistons on the centre housing via the hydraulic lines (F1) and (F2) according to the force applied to the two brake pedals. In mode 2, the equal pressure exerted either side of the valve (c1) releases the flow of oil to the hydraulic lines (F1), (F2) and (FR) (trailer brake, if fitted).