Kelsey Hayes disc brake trouble

Did you perform the proper break-in procedure with the new pads?

A 69 Dart with factory disks would have had the separate PV in the line going to the rear. The combo w/PV as fitted to later cars with single-pot brakes may not have the same proportioning built in. I installed a Summit adjustable PV when I did a drum-to-disk swap.

But usually the issue is too much rear brake, not not-enough front brake, because the PV affects the rear brakes.