73 duster front brakes locking up when coming to a stop.

NO! Oh wait, I read your post wrong; So by your description, it is plumbed correctly.

The top of the Combination Valve is for the front brakes; it's just a splitter. Fluid comes in on the end and splits to the left and right calipers. .
The bottom contains the Proportioning valve, and has a single outlet routed to the rear brakes. The P-valve does two things, it delays the brake pressure thu itself, to allow the front brakes to get energized first. and then, it reduces the line pressure to the rear, to prevent them from doing any real work. By design, they are limited to doing about 15% of the work ........................

However
do not confuse the lines going from M/C to the C-valve!
On the M/C the frontmost line is for the rear brakes. From the factory, these two lines have different fittings which makes it impossible to reverse them.
Also
aftermarket or replacement M/Cs often have "residual valves/aka hold-off valves" installed into the ports. That's fine for the port that feeds the rear brakes, but NOT FINE in the front brake system. If yours has one in the rearmost port that feeds the front brakes, it has to be defeated, else the pads will drag on the discs. Don't bother looking for it yet, as you will know right away when the front brakes start working by the fact that the brakes will not release properly.

The test for front brake operation is easy;
just get the front wheels off the ground, then jam a stick between the applied brake pedal and some anchor like the seat or the steering wheel, or just get a helper to stand on the pedal. Then go try to turn the wheels. Obviously, if they are not locked up, then they are not working.
The why of it gets a lil more detailed.

BTW-1
AFAIK, on a Mopar, the guts of a drum/drum M/C and the guts of a Disc/drum system are the same.
The differences are purely between;
the sizes of the reservoirs and
the installation of the residual valves, and
the pushrod retainer, if so equipped.
In other words, any M/C that bolts on to your car, can be made to work, relatively easily.

BTW-2
If the rear brakes on a disc/drum system are bled first, it will make it nearly impossible to bleed the fronts. This is because of the way the M/C is designed.
Once the chamber between the Power-pistons is full of fluid, the frontmost piston is coupled to the brake pedal.
Whereas the rearmost Power-piston (for the front brakes) is designed to slide past the exit port, before sending fluid out to the calipers. So if the front brakelines are empty, whatever fluid moves into the line during the bleeding operation, when the rear lines are already full and sealed, tends to just shuttle back and forth, and never actually transmits any hydraulic pressure. Thus the front brakes cannot work.
There are only two ways around this.
1) The rear line has to be opened to allow the brake pedal to travel fully to bottom the Power-pistons or
2) the rear drums have to be removed to allow the M/Cs some extended travel. If you do this tho, then you gotta not pump the pistons out of the W/Cs making a mess. After every pump, or maybe every second pump, you gotta allow the return springs to park the shoes.
The easiest by far is to unscrew the rear brakeline from the M/C and reinstall the "bench-bleeder", until after the front brakes are bled....... and known to be working.

Please note, once the two systems are both bled and functioning as designed, the discs require very little fluid movement to accomplish braking, and the sliding business that I earlier mentioned, is no longer an issue. As the front system is becoming energized, the fluid in the rear has already pushed the shoes out into contact with the drums, and so braking is ready to begin. After the pedal releases, the return springs in the rear, return the shoes to their parking spots ready for the next application. Simultaneously, "seal-retraction" and disc-runout" have knocked the pads off the rotors, and they too are ready for the next application.
If the rear self adjusting system is defeated, then as the adjustment deteriorates, rear braking becomes less and less effective, until one day, not enough fluid ever comes out to actually push the shoes out to the drums, and that end of the brakes just stops working. This does NOT affect the front brake operation except that the pedal-travel may eventually be a lil longer.

What I'm saying is that, if the guy prior to you bled the rear brakes first, as is usually done on an all-drum system, this could prevent your front system from functioning. However, this should be evidenced by an extended pedal-travel, which you did not mention. The pedal will still be hard after the sliding system has been superseded by the extended pedal-travel.
This is a part of the dual-braking system design, which still gives you rear brakes after the front system has failed, and is normal, in the "failed mode". You just have to be aware of this idiosyncrasy during bleeding.