Brake master cylinder, location / design / function

Brake master cylinder with two central valves

The brake master cylinder has one integrated central valve each in the floating piston and primary piston, which render the expansion bores in the two brake circuits unnecessary. Their function is performed by the central valves. The central valves ensure that during the control phase the primary cup seals on the primary and floating pistons are not damaged on the expansion bores by the pressure surges produced as ABS control processes commence.

S42.30-0032-06

1 Cylinder housing

2 Compression spring

3 Cup seal

4 Cylindrical pin

5 Reservoir plug

6 Floating piston

7 Sealing ring

8 Compression spring

9 Stop plate

10 Locking ring

11 Inner primary piston

12 Clamping sleeve

13 Primary piston

15 Dust boot

16 Secondary cup seal

17 Isolating cup seal

18 Valve spring

19 Valve gasket

21 Central valve

Function description

The slotted floating piston (6) is carried in the cylinder bore by a cylindrical pin (4). The cylindrical pin (4) acts as a stop for the floating piston (6), opens the central valve (A) and simultaneously acts as a replenishing port. The primary piston (13) contains a floating inner piston with a central valve. The valve spring (18) holds the inner piston (11) in the basic position (the central valve (B) is open).

When the brakes are actuated, the inner primary piston (11) is pushed to the left against the pressure of the valve spring (18), causing the valve (21) and the valve gasket (19) to come to rest against the crown of the primary piston. The central valve (B) is closed and the pressure build-up in the pressure chamber of the primary piston commences. This pressure build-up moves the floating piston (6) to the left; the valve stem lifts off the cylindrical pin (4) and is moved to the right by the valve spring (18) until the valve gasket (19) comes into contact with the floating piston (6). The central valve (A) is closed and the pressure builds up in the pressure chamber of the floating piston.

When the brake pedal is released, the two pistons (6 and 13) are pushed back to their starting positions by the force of the compression springs (2 and 8); the valve stem of the central valve in the floating piston (6) runs up against the cylindrical pin (4) and opens the central valve.

At the same time the stroke of the floating piston is limited as the piston reaches the cylindrical pin. As the force of the compression spring (8) is greater than that of the valve spring (18), the primary piston (13) runs back only as far as the stop plate (9).

The force of the valve spring (18) then moves the inner primary piston (11) to the right. The central valve (B) is open.

 It is not possible to repair the brake master cylinder.

 If the expansion reservoir is to be reused, it must be thoroughly cleaned with spirit.

 The reservoir plugs (5) must be replaced.

 The gasket between the cylinder and the brake unit must be replaced and correctly fitted.