When setting up a brake system, it is very important to use
the correct valves. If the master cylinder reservoir is located lower than
the wheel cylinders or calipers then you should have residual pressure valves.
Use a 2psi valve for disk brake calipers and 10psi valve for drum brake
wheel cylinders. This will maintain 2 or 10psi between the caliper/wheel
cylinder and valve, which is enough pressure to keep the brake fluid from
flowing back from the wheels and leaking past the reservoir vent and on
to the ground. The second function of the residual pressure is a slight
preload on the brakes keeping them at the ready.
A combination valve is several valves in one. There is a brake light warning
switch and isolation valve, a metering valve for the front brakes, and a
proportioning valve for the rear brakes. The brake light warning switch
also known as a pressure differential switch is part of the isolation valve.
The isolation valve is controlled by the front and rear incoming brake pressure.
The valve has incoming brake pressure acting on each side of a piston. If
the pressure on one side of the piston is more than the other side, the
piston will start moving toward the lower pressure. At a predetermined point
of piston movement, the brake light warning switch is triggered. If the pressure difference continues,
the piston will move far enough to completely stop fluid flow to the side
with the lower pressure. At this point, the piston will not return to center
until the valve is disassembled and reset. Heres an example: Lets
say one of the front brake hoses were to burst. The lack of front brake
pressure would have caused the warning light to come on, and the isolation
valve would stop the fluid flow to the front brakes. At the same time the
rear brakes are still working, and there is only a minimal loss of brake
fluid. |
The metering valve causes a slight delay in the front
brakes. The valve stops fluid from moving until it is above a preset pressure
(75 to 150psi) then opens and fluid flow is normal. The metering valves
purpose is to have the rear drum brakes build enough pressure to overcome
the return springs allowing the rear shoes to engage the rear drums at the
same time the front brake calipers engage the front discs.
The front to rear brake balance is partly controlled by the proportioning
valve. The proportioning valve has two functions. First it will reduce the
rear brake pressure that exits the valve. Second, it limits the maximum
pressure. If the rear brake pressure coming into the valve is below a preset
pressure, typically 500psi, there is no difference in the pressure exiting
the valve. The pressure required for the valve to start reducing pressure
is known as the split or knee point. After the split point the pressure
leaving the valve will be less than the pressure entering the valve. When
the output pressure has reached a preset maximum point the valve will close
and prevent the rear brakes from getting any more pressure. Lets look
at an example where the brakes are applied in a hard stop. The pressure
will rise front and rear equally to about 500psi after that the rear pressure
will rise at about half the rate the front does. At 1000psi at the front
brakes there will be about 750psi to the rear brakes. (The first 500psi
is equal; increasing the front another 500psi will increase the rear 250psi.)
Once the rear brakes reach there maximum pressure the front can continue
to rise without the rear rising. The limiting the rear maximum pressure
prevents the rear drums from being damaged by too much pressure and helps
control rear wheel lock up. The proportioning valves works together with
the isolation valve. If the isolation valve cycles to prevent the front
brakes from getting pressure, the proportioning and limiting functions will
be bypassed. If the front brakes failed there would be no need to balance
the braking forces front to rear. |