Brake servo and master cylinder

- The brake servo is located between the brake pedal and the master cylinder. It is fitted at the rear of the master cylinder and uses pressure from the 17 bar circuit to power assist the master cylinder piston.

Identifying ports and ports on master cylinders 

A : Control of the trailer braking arrangement
B : Supply of brake master cylinders from tank
C : To main brake
D : Pressure balancing valve
E : Trailer brake pressure chamber
F : Trailer brake valve

Identifying ports and channels on the Brake servo 

A : 17 bar supply
E : To housing return
T : Piston chamber (10)
1 . Functioning of the brake servo 

Rest position
- Piston (10) is pushed to the right by the action of spring (16).
- 17 bar pressure enters port A until it reaches groove (2) where it is stopped by spool (6). The chamber T is linked to the return E via hole (9), the central channel (5) and open port (1).

Operating position
- When pressing the brake pedal, a force is applied to spool (6) via piston (7) and rod (8). The spool (6) is pushed to the left and closes the link to the tank via supply port (1). Hole (3) is placed in communication with groove (2). The oil under pressure arrives in the supply slot at the rear of the piston (10) via channel (5) and hole (9) and enters in front of the piston until an equilibrium is obtained between the resistance of the master cylinder, via thrust-rod (17) and the force applied to piston (10).
- Pressure acting on the face of the piston (7) produces a reaction force due to the effort of the push rod (8). This equilibrium gives a waiting position for which the slightest change in force at the pedal turns into an increase or decrease in pressure until the equilibrium is newly restored. The machine's amplification factor is the ratio of surfaces of pistons (7) (10) 1/4-4.

Maximum braking position
- In this position, the groove (2) and hole (3) are entirely in communication. The spool (6) is thrust up against the pad (11), and total possible pressure (17 bar) is applied to the rear of the piston (10), and maximum assistance force is obtained. The amplifier thus reaches saturation and an increase in force on the master cylinder is not possible except by pressing the pedal harder.

- The brake servo does not require servicing. Any defective assemblies should be completely replaced.

Functioning of the master cylinders with trailer braking

Unlatched pedals 

Tractor braking and balancing
- The force applied on a pedal and transmitted by rod (17), via the brake servo, moves piston (4) along the bore of the active cylinder. Valve (1) closes the supply port, the balancing valve (3) then opens and oil under pressure is directed towards ports C and D. Through port D, the pressure closes valve (3) on the passive master cylinder.

Trailer braking
- The action on piston (4) also produces a movement of piston (6) and the closing of valve F. The oil contained in chamber E is directed to chamber E’ of the passive master cylinder and towards port B’ via the intermediary of the open valve F’. The valve F and piston (6) assembly rapidly abuts against the bottom of the active master cylinder.
- In this position, only the tractor brake is activated, the trailer is not braked.

Latched pedals 

Braking and pressure balancing
- The force applied to both pedals and transmitted by rods (17) through the intermediary of the servo, moves pistons (4) along the bores of the two master cylinders. The beginning of the travel of each piston, while displacing a certain volume of oil, closes the two F valves. The simultaneous movement of pistons (4) opens the balancing valves (3) and stops them from closing again. A balancing of pressures is carried out in the two master cylinders by way of the communicating pipe screwed into port D. The oil under pressure is also directed to tractor braking port C.

Trailer braking
- The continued action of pistons (4) (6) closes valves F and F’. Chambers E and E’ which are pressurised, supply the trailer braking valve via ports A and A’.
- In the case of an incident involving the tractor’s braking circuit, the trailer will remain braked even though the tractor is not.
- In the case of a failure of the trailer braking valve or its channels, the tractor will remain braked.
In versions without trailer braking, the master cylinder does not have port A, valve F or piston (6).

Adjusting the servo brake rod 

Measure the position of master cylinder piston (4) in relation to the flange, dimension A. When measuring this dimension do not compress spring (5).
Smear the threads of the push rod nut with Loctite 542.
Place the push rod in position in the brake assistance servo.
Measure the position of the end of the push rod (17) in relation to the flange of the servo brake, dimension B. When measuring the dimension take care not to compress spring (16) on piston (10).
Correct the length of the push rod so as to obtain a clearance of J = 0.5 ± 0.25 mm between push rod (17) and piston (4) on the assembly of the master cylinder and brake servo. Proceed in the same manner for the second servo brake.

Adjusting the brake pedals 

If disassembled, smear the pins (5) (6) with molybdenum bisulphide grease.
Smear the threads of yoke (1) with Loctite 542 and screw it onto rod (8).
Fit the return spring (3).
Adjust yoke (1) by turning rod (8) so as to obtain the dimension Y between support (4) and the centre of pin (6) in the yoke.
Y = 113 ± 0.5 mm without brake servo
Y = 139 ± 0.5 mm with servo brake
Proceed in the same way with the second yoke. Press each pedal ensuring that it goes through its full travel. Check that the pedals return freely to their initial position. Check that the pedal latch functions normally without any hard points.