[SIZE=+1]Metering Valve[/SIZE] The metering valve section of the combination valve is required on cars that have disc brakes on the front wheels and drum brakes on the rear wheels. If you have read How Disc Brakes Works and How Drum Brakes Work, you know that the disc brake pad is normally in contact with the disc, while the drum brake shoes are normally pulled away from the drum. Because of this, the disc brakes are in a position to engage before the drum brakes when you push the brake pedal down. The metering valve compensates for this, making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached. The threshold pressure is low compared to the maximum pressure in the braking system, so the drum brakes just barely engage before the disc brakes kick in. Having the rear brakes engage before the front brakes provides a lot more stability during braking. Applying the rear brakes first helps keep the car in a straight line, much like the rudder helps a plane fly in straight line. [SIZE=+1]Pressure Differential Switch[/SIZE] The pressure differential valve is the device that alerts you if you have a leak in one of your brake circuits. The valve contains a specially shaped piston in the middle of a cylinder. Each side of the piston is exposed to the pressure in one of the two brake circuits. As long as the pressure in both circuits is the same, the piston will stay centered in its cylinder. But if one side develops a leak, the pressure will drop in that circuit, forcing the piston off-center. This closes a switch, which turns on a light in the instrument panel of the car. The wires for this switch are visible in the picture above. [SIZE=+1]Proportioning Valve[/SIZE] The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes. The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located. If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.
Thanks! for posting this, going to print and save. Working on 51 Merc recently purchased and brakes are on list of things to fix. John
Nope, not correct. They perform two totally different functions and were both included in many combination valves. I don't know where your information is coming from, but some of it is inaccurate.
Is there an advantage to running a proportioning valve on a stock single-piston master cylinder system with drum brakes all around? Eventually I will upgrade, but at the moment I have stock 1954 ford brakes. But I do already have a brand new proportioning valve sitting around that I could install, if it would help. If so, where is the best place to install it on a system like this? Most info I find is regarding dual-diagonal brake systems,or front disc/rear drum systems.
No need for a metering valve with a proportion cable. Proportion valve ha metering function. Posted using the Full Custom H.A.M.B. App!
I have drums on my 53 no proportion valve and they work fine. But I know there limits. U can get adjustable proportion valve for the rear and tune it to keep the rears from locking on hard stops. Posted using the Full Custom H.A.M.B. App!
Thanks for the refresher. I understand the metering valve and the pressure differential switch. However, regarding the proportioning valve, I thought the rear wheel cylinders were smaller to compensate for the weight transfer, so no proportioning is needed?
If you read my first post it breaks down the proportioning or combination valve and its function. Prior this version the metering valve was separate. In a common disk drum setup you have the dual master cylinder to combination or proportion valve to distribution blocks to brakes. Posted using the Full Custom H.A.M.B. App!
I just converted my 59 Mercury to dual MC, all drums. There were no valves of any kind before, do I need to add a proportioning valve now?
As for the discussion on Metering, anti-dive valves vs prop valves. Depending on the design of the car, some cars did have both a proportional valve and metering(anti-dive) valve. GM used this in their combination valves on non abs to at least the late 80s/early 90s G/F/B bodies. Whereas Ford did not use a metering valve in their combo valves on their midsize Fox-Ch***is based cars ('79-'04 Mustangs, Fairmont, T-bird '80-88). I am not sure what Mopar did with their cars/trucks. Nope. All you did was divorce the system with a two circuit twin piston MC, and now have the added safety of a split system. Although hydraulically it will still work the same as when it was a single piston MC. The drum size and wheel cylinder size are what determines the bias in the system. As long as you are using the same size wheel/tires, or at least same size front to rear, the bias will remain the same. Bias can be changed by changing a wheel cylinder size, drum diameter size, or the aforementioned change of wheel diameter and/or tire compound(grip).
I don't think a person can go over this information enough. Thanks for your input and time hoodprop, some of us have to digest this more than once to retain. Sonny
When I was building my cars I worked with two companies, Master Power Brakes, 888-351-8781 and Godman Hi Performance, 901-382-7404. All these guys do is brakes and will walk you thru everything, both my cars stop like modern cars.
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