The hydraulic tandem master cylinder is mounted together with the air dual-actuator on the outside of the chassis sidemember, immediately behind the cab.
The cylinder contains two spring-loaded pistons, each fitted with rubber seals. Two detachable plastic reservoirs, each containing a filter, are mounted above tip-type recuperating valves. The recuperating valves are self-contained and interchangeable, and are screwed into tapped bosses in the cylinder so that the valve spindles project into the cylinder bore. The valves are held open by the piston flanges when the brakes are off and close under the action of their springs as the pistons move when the brakes are applied.
Two adaptors screwed into tapped bosses in the side of the cylinder retain spring-loaded check valves and provide attachment for the front and rear hydraulic brake pipes.
Removal:-
1 Disconnect the hydraulic pipes from the master cylinder.
2 Remove the nuts securing the master cylinder and actuator to the support and withdraw the master cylinder. Note the number of shims between the attaching faces as they control the clearance between the end of the actuator pushrod and the master cylinder primary piston.
Dismantling:-
1 Unscrew the reservoir caps and drain off the fluid.
2 Remove the reservoir filters using a length of welding wire shaped with a bend at the end.
3 Unscrew the reservoir retaining nuts and with draw the reservoirs, washers and sealing rings.
4 Unscrew the recuperating valves and remove the sealing rings. Do not dismantle the valves unnecessarily. If required, the valves may be dismantled by removing the spring retainers.
5 Remove the piston stop bolt at the base of the cylinder.
6 Remove the adaptors and sealing rings and withdraw the check valves and springs. Remove the front plug and sealing ring.
7 Remove the circlip at the rear of the cylinder and withdraw the pistons and springs. When withdrawing the secondary piston, use a thin feeler blade to assist the rear seal in passing over the the circlip groove.
8 Remove the perforated plate from the front end of the cylinder bore.
Inspection:-
1 Clean all parts with alcohol (methylated spirits).
2 Inspect the recuperating and check valves for damage or corrosion.
3 Examine the cylinder bore and pistons for ridges or scores.
4 All rubber seals should be renewed whenever the cylinder is dismantled.
If the seals show signs of swelling due to mineral oil contamination, the remainder of the brake system should be checked and flushed out with alcohol (methylated spirits).
Reassembly:-
Note the following:-
1 Smear the cylinder bore and all components with clean brake fluid.
2 Pack the rear seal of the primary piston with the approved grease.
3 Install the pistons and piston seals. Use a thin feeler blade to assist the seals in passing over the circlip groove.
4 Use new sealing rings when assembling the checkvalves to the springs.
5 Assemble new sealing rings to the recuperating valves and install the valves whilst holding the pistons depressed. Check that the movement of the pistons actuates both valves.
6 Position the rubber washers on the recuperating valves and the copper washers under the reservoir retaining nuts.
Installation:-
1 Before installing the master cylinder, determine the thickness of shims required between the cylinder and the support. This may be achieved as follows:-
With the actuator located in the support assemble the shims removed and gauge VR2096, as shown in N25. Add or remove sufficient shims until the pushrod is flush with and not more than .005in below the top of the gauge. As a final check the shims should be compressed by securing the gauge with three nuts.
2 Smear the recess in the primary piston with the approved grease.
3 Refill the reservoirs to the indicated level and bleed the brakes.
Illustration of checking gauge VR2096:-
http://i24.photobucket.com/albums/c1...n/IMGP8510.jpg
Illustration of the master cylinder:-
http://i24.photobucket.com/albums/c1...n/IMGP8512.jpg
(Without the checking gauge VR2096 - which will undoubtedly be the case - the depth of the depression in the master cylinder piston must be calculated such that there is no free play or lost movement when the cylinder is attached to the air actuator, nor must the actuator pushrod cause movement of the master cylinder piston before air is applied - although it might be necessary for trial and error of the number and thickness of the shims until satisfactory operation is accomplished.
It might be advisable to measure the dimensions of the air actuator pushrod and the depression of the removed master cylinder piston to determine how much clearance or preload is required.)