I keep pressure on the pedal it will slowly go down to the carpet

I have had an on going problem with the brakes, I changed master cylinder, power booster shoes and pads and still the brakes are not great. No matter how slow I am going and how hard I press the pedal they will not lock, when going down a steep incline they have a hard time slowing the car down. Yesterday when pulling out of the garage the rear wheels locked when I just touched the pedal. I have been told to check the proportioning valve, but I thought this was just for emergency braking. Any info would be appreciated.

The Proportioning valve is designed to keep the rear brakes at a lower pressure than the front, and also to provide pressure to either the front or rear, if one system fails. If the pressure in the front drops too low, the rear brakes will take the full force of the braking action (and lock up). Some are also designed to reduce the amount of air let back into the system if a leak should occur. Cherokees are a very heavy vehicle, and the braking system on the older ones were hard to lock up. With the recent problem of having the rear brakes lock up on you, I would suggest that you very carefully check all of your lines for pinhole leaks. You can also upgrade your rotors to drilled and/or slotted, to help to improve the performance. Also check for proper adjustment of the rear brakes and make sure all caliper/shoe contact points are smooth, well polished, and lubricated with a grease designed for brake systems. Use 120 grit sand paper to break any glaze on rotors, or have them turned if they show wear.