You must have a proportioning valve for a disk/drum setup. Best is an adjustable valve since the factory ones are for a specific setup. Even changing tires or wheel sizes could change the ideal ratio, or shifting weight between front and rear (different engine, AC, different seats, ..). The proof is that many pickups and vans came with a level-sensing proportioning valve (arm senses deflection of rear springs). Ideally, you will run skid-test in a wet parking lot and set the proportioning so the fronts skid just before the rears. If the rears lock up first in an emergency, the rear end can swing around. A basic adjustable valve is $30. You can also buy with an imbalance warning switch for ~$70 (also adjustable). Those might also have the front metering valve, as in the factory "combination valve".