I found this FAQ online about adj prop. valves. I have a 14 bolt and was going to go with disks and a willwood adjustable valve. But I also think the combination valve in my truck is bad. Even with drum brakes the rear tends to lock up. Can I bypass it all together? From what this is saying I would guess no? What does a proportioning valve do? A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions. What are the symptoms of a bad proportioning valve? If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces. What does a metering valve do? A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear. What are the symptoms of a bad metering valve? Your car will nose dive and the front pads will wear too fast. What does a residual valve do? There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum brakes to give a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master. What are the symptoms of a bad residual valve? The brakes will be very spongy and you will need to pump the pedal to get good brakes. What is a combination valve? A combination valve incorporates metering and proportioning into one valve providing all necessary valving for disc/drum systems. See below Should I use an adjustable proportioning valve? Not if you can help it. The adjustable valve will only provide the proportioning function and not the metering that is needed.