Proportioning valve wouldn't have any effect on the front brakes.
The prop valve reduces the increase in hydraulic pressure to the rear brakes starting areound 350 psi.

It's more likely that a problem with the from brakes involves a metering valve.
Metering valve is supposed to hold off application of presure to front brakes under very light braking.
They were used on E-bodies, at least in '70.
See here
http://www.imperialclub.com/Repair/Lit/Master/274/page09.htm

Also possible that something in the safety switch & distribution block.
Same link will get you rebuild kit info for them.

It looks to me like '72 model year e-bodies got a combination meter valve, safety switch and proportioning valve.
see here
http://www.imperialclub.com/Repair/Lit/Master/293/page10.htm


Because the metering valve holds off hydaulic fluid moving to front brakes under light braking, some form of pressure bleeding or pumping the brake pedal will be needed.
So check out the recomendations for bleeding those if you haven't already.

Final thought is there a residual valve somwhere in the front lines.