Main braking circuit of Dyna VT transmission

Tractors with a DYNA VT transmission are fitted with a hydraulic braking system operating on the high pressure auxiliary circuit for the rear brakes and on the transmission low pressure circuit for the front axle shaft brake. They are both controlled by a braking valve activated by two pedals that can operate coupled or uncoupled. The braking valve, located at the front of the pedal, receives and directs pressure to the various components. It controls:
- The right- and left-hand brakes located under the rear axle, and operating simultaneously or separately,
- the front axle shaft brake control,
- the trailer brake control via a pressure sensor located on the valve.

Rear brakes
Located in the rear axle, the two disc brakes are independent. Each brake is controlled hydraulically by a cylinder, or mechanically by the ParkLock ram. The braking valve is supplied by the make and break valve located under the distribution block with an accumulator that maintains a pressure reserve of 135 – 165 bar. The braking valve includes a pressure relief valve (0-70 bar). The pressure is distributed to the brake by two spools located on either side of the pressure relief valve.

Front axle shaft brake
The front axle brake is located at the front of the gearbox. It is a hydraulically controlled multidisc brake.
The main braking valve controls a power-assisted braking valve which operates the front brake under a 18 bar max. pressure (transmission low pressure circuit).

Trailer brake
The trailer brake is controlled electro-hydraulically. A pressure sensor informs the Autotronic 5 calculator of the pressure in the braking circuit. This then activates a proportional solenoid valve that controls the trailer brake valve via the transmission 18 bar circuit.

Characteristics of braking components

Braking valve
- Braking pressure: 0 to 70 bar.
- Working travel: 16 mm
Rear brakes
- Piston diameter: 50.8 mm
- Piston stroke: 3.7 mm
- Maximum volume used by piston: 12.25 cm3 on each side
Front brakes
- Piston diameter: 62 mm
- Piston stroke: 6.1 mm
- Rear brake / front brake pressure ratio: 2.04 bar
- Maximum braking pressure: 18 bar

Braking valve control

Parts list 
(1) Main braking valve block (2) Braking pressure regulator (3) Left-hand brake control (4) Right-hand brake control (5) Pedals (6) Bearing (7) Bearing (8) Bearing (9) Bearing (10) Sensor (11) Spring

Identification of ports 
(P) High pressure supply
(T) Tank return
(X) Control of front axle shaft brake and trailer brake pressure sensor
(F1) Left-hand brake supply
(F2) Right-hand brake supply

Braking control
The pedals are pressed against the braking valve rods via bearings (6) (7) (8) (9). Each pedal simultaneously controls the pressure regulator and corresponding spool valve (3) or (4).

Relations between the different tractor braking circuits

The braking circuits of the rear axle, front axle and trailer are hydraulically or electrically linked. These circuits must react simultaneously to a single action on the brake pedals.

Parts list 
(P1) High pressure variable displacement pump
(P2) Transmission low pressure hydraulic pump
(PB) Trailer brake and ParkLock priority valve
(CD) Make and break valve
(TV) Trailer braking valve
(BV) Tractor braking valve
(EV) Trailer brake control proportional solenoid valve
(AC1) Accumulator (ParkLock)
(AC2) Accumulator (rear brakes)
(TB) Trailer brake
(BR1) Rear brake
(BR2) Rear brake
(BP1) Pedal
(BP2) Pedal
(M) Pressure sensor
(FD) Front brake control valve
(BF) Front brake
(AUX) To non-priority components
(AUTO 5) Autotronic 5 calculator
(PARKLOCK) ParkLock assembly (ram and control)

Action on tractor brakes

Pedals coupled
When one of the two pedals (BP1) and (BP2) is activated, the pressure regulator located in the braking valve regulates the pressure coming from the make and break valve (CD) and accumulator (AC2). The braking valve then distributes this pressure to the rear brakes (BR1) and (BR2).
Simultaneaously, the braking valve sends a pressure signal to the spool valve (FD) and pressure sensor (M). The spool valve (FD) controls the front axle brake (BF) with pressure from the pump (P2) (transmission 18 bar circuit). The sensor (M) allows the Autotronic 5 to control the trailer brake via the proportional solenoid valve (EV).

Pedals uncoupled
If the driver presses on just one pedal, this acts on the braking valve pressure regulator and on a brake spool valve. The pressure is directed to the corresponding brake. The “&” valve in the braking block ensures no pressure signal is supplied to the spool valve (FD) or sensor (M). The front brake (BF) and trailer brake (TB) are only activated when both brake pedals are simultaneously activated.

“&” Valve
The “&” valve is fitted in the braking block. It allows pressure to be supplied to the trailer brake and front brake only if the two pedals are activated. The pressure directed to the rear left- and right-hand brakes must be identical to enable operation of the front brake and trailer brake.


Parts list 
(1) Tractor braking valve
(2) Pressure relief spool
(3) Right-hand brake spool
(4) Left-hand brake spool
(5) Right-hand brake
(6) Left-hand brake
(7) & Valve
(8) Accumulator (braking circuit)
(9) Accumulator (ParkLock circuit)
(10) Transmission low pressure circuit pump
(11) Front axle shaft brake control spool
(12) Front axle shaft brake
(M1) Braking circuit load pressure switch
(M2) Braking circuit load pressure switch
(TR 34) Trailer braking pressure sensor
(A) Trailer brake control
(B) To ParkLock
(C) Braking circuit supply pressure from make and
break valve (135 - 165 bar)

Rear brake cylinders

The rear brake cylinders are located on the rear axle, beside the trumpet housings.

The rear axle braking system consists of a brake cylinder (1), a piston (2), and a rod (5) that acts on an assembly (3) comprised of discs (6) and intermediate plates (7) (see chapter 6). The brake disc assembly is fitted before the final drive unit (4) in the rear trumpet housing. The piston acts on the rod (5) whose triangular end allows the two intermediate plates (9) and (10) to swivel in relation to each other. When swivelling, the intermediate plates (9) and (10) are kept separate by balls (8). Return springs (18) restore the intermediate plates (9) and (10) to position when the brakes are released. The port (16) is linked to the bleed circuit by a pipe.

Braking can be activated hydraulically (brake pedals) or mechanically (ParkLock).

Pedal braking
The braking pressure (coming from the braking valve) arrives at the cylinder via the port (14). The piston (2) is forced down by the pressure, driving down the lower part of the piston (17) and rod (5) and activates the braking mechanism.

Mechanical braking (ParkLock)
When the brakes are activated by the ParkLock ram, the levers (18) activate the cams. As they rotate they drive the lower part of the pistons (17) and rod (5) and activate the braking mechanism. When the lower part of the piston (17) has lowered, the spring (15) keeps the upper part of the piston (2) at the top of the cylinder.

Removing the universal joint shaft brake 
The front axle shaft brake is located at the front of the gearbox.

The front axle shaft brake operates in a similar manner to the rear brakes. A piston pushes a rod (21) with a triangular end. The triangular part allows two intermediate plates to swivel in relation to each other, while they are held apart by balls (23).

Bleeding the brakes

The tractor brakes are bled through bleed plugs located at the rear right-hand side of the tractor beside the auxiliary spool valves:
(1) Front axle shaft brake
(2) Rear left-hand brake
(3) Rear right-hand brake
The trailer brake is bled through the bleed plug located on top of the distribution block. The brakes must be bled every 1200 hours, or when working on the braking circuit.

Bleeding method
Start the engine. Open the right-hand (3), left-hand (2) and front (1) bleed plugsPress moderately on the coupled brake pedals, with pedal travel of approximately 20 mm. Never press the pedals down more than 25 mm when the bleed plugs are open. A jet of oil under pressure will spurt out of the bleed plugs.

Let the air bleed out at the plugs.
Close the plugs again when the fluid coming out no longer shows any trace of foam or air bubbles. Release the pedals. Open the bleed plug on the trailer brake valveRepeat the operationsCheck the oil levels in the auxiliary hydraulics and gearbox.