Brake master cylinder

The force applied to the brake pedal is transmitted by the control rod (2) and moves the spool (3) in the bore of the piston (4). The oil under pressure coming from the port (a) enters the holes (10) and the groove (12). The piston (4) is then pushed to the left, compressing the oil in the hydraulic chamber (8) of the brake master cylinder, then directing it to the port (d) and the main brake piston via the FTE valve (if fitted). The screw (7) guides the piston (4) along its movement axis. Under no circumstances should this screw (7) be removed. When the brake pedal is released, the spool (3) returns to its initial position. 

The piston (4) is no longer under pressure. It therefore returns to its resting position. The valve (6) allows the pressure between the brake master cylinder hydraulic chamber (8) and the oil reserve (5) to be balanced only once the piston has returned. The port (9) allows oil flow in order to maintain a constant level in the brake master cylinder chamber (8). The oil under pressure (17 or 21 bar) in chamber (b) also fills reserve (5), passing through the bore (11). Port (c) removes any excess oil and directs it to the return.

Parts list 
(1) Master cylinder assembly (2) Rod (3) Spool (4) Piston (5) Brake system supply reserve (6) Pressure balance valve (7) Piston guide screw (8) Master cylinder hydraulic chamber (9) Port (10) Port (11) Bore (12) Groove

(a) Supply (17 or 21 bar)
(b) Piston (4) chamber
(c) To the return
(d) To the main right and/or left-hand brake via the FTE valve (if fitted)
1.Old generation master cylinders
2.New generation master cylinders

FTE valve

Parts list 
(1) Spool (2) Ball bearing (3) Plunger (4) Spring

(A) To trailer brake valve
(B) From right-hand brake master cylinder
(C) From left-hand brake master cylinder
(D) To right-hand tractor brake
(E) To left-hand tractor brake

The FTE valve (14) is an essential component of the trailer brake system. It performs a logical "AND" function, i.e. it allows oil to flow to the brake valve only when both brake pedals are activated at the same time. If only one brake pedal is pressed, the brake valve is not activated.

Pedals uncoupled 
When force is applied to only one brake pedal, the oil under pressure coming from the brake master cylinder and going to the port (B) moves the spool (1) and presses the ball bearing (2) against the spool to block the connection (D) and (A). Therefore, there is only one possible way for oil to flow from (B) to (D).

Pedals coupled 
When force is applied to both pedals simultaneously, the spools (1) are thrust against each other and compress spring (4). The plunger (3) then forces the ball bearings (2) apart, lifting them from their seats in the spool. In this way all the ports are directly connected, i.e. there is a braking pressure supplied to tractor brakes (D) and (E) and to the brake valve (A).

 Adjusting brake pedals

Preparing for adjusting 
If the pins (5) and (6) are removed, smear them lightly with molybdenum disulphide or AS767 grease or equivalent. Screw the counter-nut and clevis (1) onto the rod (2). Install the return spring (3).

Adjust clevis (1) to obtain dimension X = 135 ± 0.5 mm between the support (4) and clevis attachment pin. Lock the counter-nut.
Carry out the same procedure on the second clevis and ensure that the locking latch (1) operates smoothly.
Check pedals for correct operation. Ensure they are at the same height when not activated and that they return freely to their initial position.