Despite opinions to the contrary, heavy pulls are almost never due to having too heavy sear springs, they're due to incorrect angles on the full cock notch and nose of the sear. The nose of the sear should be clean with sharp angles as should the full cock notch. If the lock won't hold at full cock, the angles on the tumbler and nose of the sear are incorrect or worn. The sear will, of necessity, rub on the lock plate at least a little but this wear mark should be pretty even and not just at the tail of the sear. With a two screw bridle you can overtighten the sear screw which will make the sear hard to move and result in a heavy pull or cause the sear to not engage when you pull the lock to full cock.
Without having the lock in hand, it's difficult to diagnose the problem - the lock could also just be so worn that it's barely serviceable.
Tom