No, the valve never changes the breakpoint pressure itself.
The break point pressure is the thing that is being adjusted, when one turns the knob or moves the lever on the proportioning valve.
In the image that I linked above shows the behavior of the proportioning valve when the break point pressure is adjusted to one of seven different settings. All of the curves are overlaid on one graph. When the proportioning valve is set to 100psi:
If you apply 100psi of brake fluid pressure to the inlet of the valve, there will be 100psi at the output port. If you then apply 200psi to the inlet port, you get 150psi at the outlet port. If you apply 300psi to the inlet port, you get about 190psi at the outlet port. So on and so forth.
If you set the breakpoint pressure to be 650psi:
And apply 700psi to the inlet port, you get about 680psi to the outlet port. If you apply 800psi to the inlet port, you will get 730psi to the outlet port.
When the breakpoint pressure is set to 100psi, that will be when the knob is set to minimum rear brake. When the valve is set to 900psi, that will be when the knob is set to maximum rear brake.
The job of the proportioning valve is to take a system which is parabolic (nonlinear) and try to emulate this behavior with only linear elements. If you look at post 1 and 12 of the link below, it gives an overview of this.
Regarding the front brake calipers. If the caliper body is steel, they are the 1994-98 models. If they are aluminum, then they are the 1999-2004 models. You can try to stick a magnet to them.