The proportioning valve (P valve) is placed between the brake line master cylinder and the rear wheelÅfs wheel cylinder. This device obtains proper braking force to shorten the braking distance by approaching the ideal front and rear wheel braking force distribution to prevent the rear wheels from early lock up during emergency braking (when the load transfers to the front), etc. When the distribution is like that shown by (a), the braking force becomes large causing the rear wheel braking force to become too much larger than the ideal curve, which makes it easy for the rear wheels to lock up and destabilize the vehicle. In addition, when the distribution is like that shown by (b), the overall braking force becomes small, which allows the front wheels to lock up easily and cause loss of steering control.
The P valve consists of the following components.
(1) Valve body
(3) Valve seal
(4) Compression spring
(5) Cylinder cup
The hydraulic pressure generated by the master cylinder acts on the front and rear brakes. The rear brakes are controlled so that the hydraulic pressure is kept the same as that of the master cylinder up to the split point and then is made lower than that of the master cylinder after the split point. The P valve operation condition is shown below.
1. Operation up to the split point
The spring force pushes the piston to the right. The hydraulic pressure from the master cylinder passes through the gap between the piston and the cylinder cup to apply an equal force to the front and rear wheel cylinders. At this time, a force works to move the piston to the left utilizing the difference in the pressure reception surface area, but it cannot overcome the spring force, so it does not move.
2. Operation of the split point
When the hydraulic pressure applied to the rear wheel cylinder increases, the pressure pushing the piston to the left overcomes the force of the spring causing the piston to move to the left and close the fluid circuit.
3. Operation after the split point
When the hydraulic pressure from the master cylinder increases even further, this increase in pressure pushes the piston to the right to open the fluid circuit. When this happens, the hydraulic pressure to the rear wheel cylinder begins to rise and the pressure pushing the piston to the left begins to increase, so before the hydraulic pressure to the rear wheel cylinder rises completely, the piston moves to the left and closes the fluid circuit. This valve operation is repeated to keep the hydraulic pressure on the rear wheel side from increasing more than that on the front wheel side.
4. Operation when the pedal is released
When the hydraulic pressure from the master cylinder decreases, the fluid on the rear wheel cylinder side passes through the outside of the cylinder cup and returns to the master cylinder side.