I am thinking that RichieHD has the right idea.
The brakes are dragging because the Compensating Ports are blocked in the master. Just a guess, but Thats what I am thinking.
The booster has a pushrod that sticks into the masters plunger. If the pushrod is too long, when you bolt down the master cylinder, it starts to push the piston into the bore. Inside the reservoir of the master, you will see a couple holes at the bottom. Those holes allow fluid to fill the cylinder,or bore of the master. If the hole is blocked, the cylinder is hydraulically locked ,the brakes will apply due to heat, which causes the expansion of the fluid, thus pushing the caliper pistons against the rotors, applying the brakes, causing more heat, locking the brakes even tighter untill ...
Those holes , or compensating ports, have to be open to allow fluid to return to the master after applying the brakes, and also due to heat expansion . If the holes are blocked, the pushrod adjustment should be shortened.
Knowing what to look for , I look into the ports in the master, as you should be able to see the cup, or seal, on the plunger, and whether or not it is covering the port or not.
A quick 5 minute check should reveal if that is the problem.
No reason to put a prop valve in the front system.
One other thing to consider is whether or not the master is for drum/ drum system ,or even drum disc, which could have residual pressure valves in the front and or rear outlets of the master cylinder.
If you have a drum /disc master , the rear of the cylinder is usually for the front system , has a larger volume reservoir,and has no residual valve. The front half would be for the rears, and could have a residual pressure valve in it.
If that is the case, and you have the front brakes plumbed to the front of the disc/drum master....that would be the problem. The residual valve is holding pressure and causing the brakes to drag.
Your post is a little tough to follow as you stated you put on a "hydroboost brake kit". That doesnt tell much about what the braking system actually is. Maybe you could elaborate a little more.