Having done yet more reading it may not be particularly relevant if I have a combined metering, proportioning and pressure differential valve which I should have as the braking system is not the normal diagonal split circuit but split into front and rear circuits.
"Metering Valve
As a result of their design, rear drum brake shoes must move a greater distance to apply as compared to
disc brake pads. If the same pressure were applied to both the front disc and rear drum brakes, at the
same time, the front discs would “catch” much sooner than the rears and cause the vehicle to be thrown
forward. Metering valves are therefore used to compensate for this condition by blocking fluid pressure to
the front disc brakes until the rear shoes have had time to make contact with the drums.
• As the brakes are first applied, fluid pressure rises above a calibrated value (approximately 25 psi
- figure 12) which closes the metering valve stem and blocks the fluid pressure from reaching the
front disc brakes. However, fluid pressure is still applied to the rear brakes, which move the
shoes out to contact the drums.
• Once the shoes begin to contact the drums, the pressure in the rear brake system starts to rise
dramatically. After the pressure reaches a second calibrated value (about 100 psi - figure 13) the
metering valve opens and begins to apply the front disc brakes.
• As the brakes are released and the system pressure again drops below 25 psi, the valve stem
reopens to allow fluid to return to the master cylinder (fig. 14). Notice also in figures 12 through
14 that there is a difference between the valve stem and the valve itself. Each is operated by a
separate spring and has a separate function.
Note: The two pressure points (25 psi & 100 psi in this example) are calibrated based on the size
and weight of a particular vehicle. Metering valves are not universally interchangeable even
though they may appear to be identical."
Interesting stuff this.
