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. Here’s an example: Let’s 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 valve’s 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. Let’s 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.For more information contact Classic Performance Products for your free catalog or visit our website at www.classicperform.com