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Tech Talk with Master Power Brakes

Combination Valves for Classic Car Brake Systems

Posted by toby on Jan 7, 2015 2:15:36 PM

A Combination Valve, commonly called a Proportioning Valve, is just what the name implies…a valve that does a combination of things. The valves are available for both disc/drum and for disc/disc brake systems. Valves for both configurations contain a Proportioning Valve and a Brake Pressure Differential Switch while the disc/drum valves also contain a Metering Valve. What is all of this stuff and what does it do? Read on and we’ll explain…..

Proportioning Valve: This portion of the valve (seen below and labeled  A) is used to control the rear brakes. There is a pre-set portion of the valve that regulates the rear brake pressure under heavy braking scenarios and prevents the rear brakes from locking up. Without this part of the valve, the rear brakes, which require much less brake fluid pressure than the front, tend to lock-up under a quick, hard press of the brake pedal. This is a potentially scary situation.

Brake Pressure Differential Warning Switch: In the center of the Combination Valve (seen below and labeled  B) is a single wire connector that works in conjunction with a slide in the center of the valve. If brake pressure is lost to either the front portion or the rear portion of the system, the switch grounds and illuminates a warning light in the instrument cluster. This is a warning that something is wrong.

Metering Valve: At the front of the Combination Valve (seen below and labeled  C) is the Metering Valve. The Metering Valve, sometimes referred to as a Hold-Off Valve, is used to regulate the pressure going to the front wheels when the brakes are initially pressed in a disc/drum combination. The disc brakes will apply much quicker than the rear drum brakes and if pressure is not slightly held off, a nose dive of the vehicle may occur under braking.

VL3350 Email blast

Included in all Combination Valve Kits from MP Brakes is a somewhat unusual looking white plastic part. This part is a tool that should always be used when bleeding the brakes. This simple little tool will save a bunch of headaches by holding the slider in place and therefore not allowing the valve to become “tripped”. Just like a brake pressure failure will cause the slider to move, the same can happen when bleeding fluid from the master cylinder to the rest of the system. Getting a valve “un-tripped” is always a challenge, so take the extra time to insert the tool.

When purchasing the VL3360K Combination Valve for disc/drum brakes, you will have some extra parts. Don’t throw them away! Should you ever want to convert the car to disc brakes, we include a plug in the kit that will replace the Metering Valve. Simply locate the nut on the front of the valve, remove the Metering Valve and replace with the included plug. You now have a valve that will work with disc/disc combinations.

When you look at the Combination Valve, you will notice two 3/8”-24 ports that both show going to the front brakes. You don’t have to use both or you can. The choice is yours. This will be based on the plumbing in the vehicle. If you want to use just one, plug the other and then “T” the front lines later in system. If you have additional questions about Combination Valves for your classic ride, be sure to check out our Valve FAQ, send us an email, or give us a call at 1-800-381-9772.

Topics: brake pressure differential warning switch, brake system, classic cars, combination valves, drum brakes, Vintage Cars, Combination Valves, Disc Brakes, metering valves

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