Understanding a Combination Valve
Mark SimpsonDescription
Combination valves are common on most classic cars from the 1960s forward. This valve is often misrepresented as the proportioning valve, however the combination valve actually contains three distinctly different functions. Mark Simpson explains the different components within the combination valve and exactly how they function together.
Most cars built in the sixties and newer have in their braking system, a combination valve. And you know, a lot of times this gets mislabeled and people will call it a proportioning valve. But it's not really a proportional valve. It's a con, this one is actually a combination valve. And what that means is it's actually a series of three valves in one in the front here we have what's called a metering valve and that operates the front braking on a front disc brake car. The front desk will want to actually kick in before the backs and to get more equally. And you don't want your car to nose dive as soon as you apply the brakes. So, what this does is this metering valve actually slows down the fluid, going to the front braking system just a little bit and it keeps the car from applying the brakes more evenly as opposed to nosediving when it. When you initially apply the brakes in the back of the combination valve is the actual proportioning bell and what this does you know, under normal braking, it doesn't do a thing. It just, it allows brake fluid to come in and pass to the back. But under real hard braking, if in a panic stop situation this kicks in and restricts the amount of fluid going to the rear brakes in the, the reason that is is you want your front brakes to engage first before you know, otherwise, if you really lock up your rear brakes the car can go into a skid situation. So in a panic stop situation that proportioning valve kicks in and restricts how quickly the fluid gets to the rear brakes. It does not alter the amount of pressure or the amount of fluid. It just slows it down. In the middle, we have what they call the low pressure switch. And what happens here is, it's just a rod that has a detent in it. And that little groove in that rod should the front brakes or the rear brakes lose pressure. That rod will shift from one side to the other whichever side has low brakes. And basically what happens at rod then suddenly comes into contact with this steel rod and then creates an electric circuit that signals a warning light on your dash to indicate that, you know, you have a low brake pressure, either in your front or rear when bleeding the brakes that a lot of guys often forget to do this because you'll be open and bleeders on the front, and on the back, it is potential that you can actually trip this switch and it'll get stuck. They make a tool specifically to hold that rod in place. And this one here is for master power brakes. And you just insert this tool into the top before you start bleeding your brakes, firmly screw it in. And that prevents that low pressure switch from activating. When you're done bleeding, your brakes simply unscrew the tool and reinserts the switch And that's pretty much that's how this combination valve works with a better understanding of what's going on with your valve. You'll be able to, you know, diagnose problems and on these if you should ever switch to a disc disc brake situation where you have disc brakes in front and disc brakes and backs, you can eliminate this metering valve portion of this simply by removing this plug, removing the metering valve in the front and replacing that with a plug then your proportioning valve will be good for a disc disc brake conversion.
So it delays the front brakes and the rear brakes?LOL. And the bar trips the switch. Well sure it does, but isn't the important part that it maintains brakes in the half of the system that's still functioning? Honestly, I'd have to read a little more to be sure, but I'm thinking that was a lot of balogna. In an emergency if the front brakes apply first, the rear end will try to pass the front end.
I have a 71 Chevy K20 pickup. The factory comb valve only had one port for the front brakes. The brake lines split in two lower down on the frame rail for left and right front disc brakes. I replaced the comb valve with a new one that had two ports on the front for left and right front brake. I capped on of the ports an am supplying the front brakes through the one port. Will that be enough fluid and pressure to operate the front brakes or do I need to find another combo valve from that era that fits my truck. The truck isn't drivable yet so I can't road test it. Thanks, I appreciate any help. Jack
I have been having a long term problem on my Chevy 1985 C10. When you first drive it in the morning the rear brakes will lock up as soon as you press the brake. After several stops this seems to go away at least to where they don't lock up. I've changed several components but to no avail. What would be your best guess at my problem.
I have a 2000 Saturn and it keeps blowing rt read wheel cyls it's been replaced many times
Could the Combination Valve be the cause of my front disk brakes locking up upon activation...It's an 80 model 1 ton chevy truck, I have repaced the calipers and master cylinder, and when the brakes are applied, they lock up and I have to bleed them to move the vehicle...Thank You, Jerry Lindsey