See, the key word here is "hard".
Normally, that will cause the fronts to slide.
If the proportioning valve is working, it will be darn near impossible to lock the rear brakes.
Since the dash-light is not on but is known to work, then there are two immediate options to look for.
Firstly, lets prove the electrical circuit is working, then
Secondly, we'll check the plumbing.
1) open the hood and find the Safety switch on the Combination Valve and verify that the circuit works by removing the wire off it and grounding it.
2) turn on the key and look for the red light.
3) if you get the light on, then the circuit is working, and we can assume that the shuttle valve is centered.
4) that sorta points to the valve being plumbed wrong, so
5) As to the Combination-valve.
First, you do have a dual-reservoir master cylinder with TWO brake lines coming out of it right? If true
then, the Combination-Valve, is mounted on the Driver-side apron or frame-rail, close to the firewall, with the knobby-looking thing at the bottom. That thing is the Proportioning-Valve.
Now, as to plumbing.
The reservoir closest to the Firewall should be plumbed to the FRONT brakes, via the TOP of that Combination-valve
The reservoir closest to the rad, should be plumbed to the rear brakes, via the Proportioning valve, and the BOTTOM outlet should be going to the rear brakes.
There is no hydraulic connection inside the Combination valve, between the front and rear systems unless somebody took out the shuttle valve.
The front brake section is just a splitter.
If yours is NOT plumbed this way, then it is wrong and needs to be restored to factory correct.
Check it out, and lemmee know, and from your answer, we can form a new plan.