Discovery 2. Manual — part 630

BRAKES

DESCRIPTION AND OPERATION

70-11

Master cylinder assembly

Section through master cylinder assembly

1 Reservoir filler cap
2 Switch float
3 Brake fluid reservoir
4 Primary feed hole
5 Valve pin
6 Primary centre valve
7 Primary piston
8 Primary spring

9 Pressure chamber

10 Cylinder
11 Valve pin
12 Secondary centre valve
13 Secondary spring
14 Pressure chamber
15 Secondary piston
16 Secondary feed hole

The master cylinder assembly produces hydraulic pressure to operate the brakes when the brake pedal is pressed.

The assembly is attached to the front of the brake servo assembly, and comprises a cylinder containing two pistons
in tandem. The rear piston produces pressure for the primary circuit and the front piston produces pressure for the
secondary circuit. A brake fluid reservoir is installed on top of the cylinder. The reservoir is internally divided to provide
an independent supply of fluid to each brake circuit, and so prevent a single fluid leak from disabling both primary and
secondary brake circuits. Should a failure occur in one circuit, the remaining circuit will still operate effectively,
although brake pedal travel and vehicle braking distances will increase. If the fluid level in the reservoir is too low, a
float operated switch in the reservoir filler cap connects an earth to the instrument pack, which illuminates the brake
warning lamp.

BRAKES

70-12

DESCRIPTION AND OPERATION

Brakes applied
When the brake pedal is pressed, the output rod in the brake servo assembly pushes the primary piston along the
cylinder bore. This produces pressure in the primary pressure chamber which, in conjunction with the primary spring,
overcomes the secondary spring and simultaneously moves the secondary piston along the cylinder bore. The initial
movement of the pistons, away from the piston stops, closes the primary and secondary centre valves. Further
movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the
brake circuits. The fluid in the chambers behind the pistons is unaffected by movement of the pistons and can flow
unrestricted through the feed holes between the chambers and the reservoir.

Brakes released
When the brake pedal is released, the primary and secondary springs push the pistons back down the bore of the
cylinder. The rapid movement of the pistons cause partial vacuums to form in the pressure chambers, which opens
the centre valves and allows fluid to circulate unrestricted between the two hydraulic circuits and the reservoir. When
the pistons reach the brakes off position, the centre valves are held open by the piston stops.

BRAKES

DESCRIPTION AND OPERATION

70-13

ABS modulator

ABS modulator schematic

1 Master cylinder/brake servo assembly
2 Brake pedal
3 ABS modulator
4 Shuttle valve switch
5 Shuttle valve
6 Damper chamber
7 Restrictor

8 Return pump
9 Non return valve

10 Expansion chamber
11 Outlet solenoid valve
12 Inlet solenoid valve
13 Rear brake
14 Front brake

BRAKES

70-14

DESCRIPTION AND OPERATION

The ABS modulator is a 4 channel unit that controls the supply of hydraulic pressure to the brakes in response to
inputs from the SLABS ECU. The modulator is attached by three mounting bushes to a bracket on the LH inner front
wing, and connected to the primary and secondary hydraulic circuits downstream of the master cylinder assembly.
Three electrical connectors link the ABS modulator to the vehicle wiring.

Passages within the ABS modulator, separated into primary and secondary circuits, connect to the various internal
components that control the supply of hydraulic pressure to the brakes:

l

Shuttle valves and non return valves control the flow through the internal circuits.

l

Shuttle valve switches, connected in series to the SLABS ECU, provide a brakes on/off signal.

l

A damper chamber and restrictor are included in each circuit to refine system operation.

l

Inlet and outlet solenoid valves control the flow to the individual brakes.

l

An expansion chamber is connected to each circuit to absorb pressure.

l

A return pump is connected to both circuits to provide a pressure source.

The ABS modulator has three operating modes: Normal braking, ABS braking and active braking.

Normal braking mode
When the brake pedal is pressed, pressurised fluid from the master cylinder assembly moves the shuttle valves to
open lines 'A' and close the shuttle valve switches. Pressurised fluid then flows through the open inlet solenoid valves
to operate the brakes. The closed shuttle valve switches supply a brakes on signal to the SLABS ECU. If the SLABS
ECU determines that EBD is necessary, it energises the inlet solenoid valves for the brakes of one axle. The inlet
solenoid valves close to isolate the brakes from any further increase in hydraulic pressure.

ABS braking mode
When in the normal braking mode, if the SLABS ECU determines that ABS braking is necessary, it energises the inlet
and outlet solenoid valves of the related brake and starts the return pump. The inlet solenoid valve closes to isolate
the brake from pressurised fluid; the outlet solenoid valve opens to release pressure from the brake into the expansion
chamber and the return pump circuit. The brake releases and the wheel begins to accelerate. The SLABS ECU then
operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the brake and apply the
maximum braking effort (for the available traction) without locking the wheel.

Active braking mode
When ETC or HDC are enabled, and the SLABS ECU determines that active braking is necessary, it starts the return
pump. Hydraulic fluid, drawn from the reservoirs through the master cylinder, shuttle valves and lines 'B', is
pressurised by the return pump and supplied to lines 'A'. The SLABS ECU then operates the inlet and outlet solenoid
valves to control the supply of hydraulic pressure to the individual brakes and slow the wheel(s).