It has been a while since I have flown the PA28R, PA34 & PA44 series of Piper aircraft, but from memory on all these aircraft the undercarriage was held up by hydraulic pressure only (no up locks).
If you lose hydraulic pressure the undercarriage will extend!
The best place to confirm this is a schemetic of the electro hydraulic power pack in the Information/Flight manual from Piper.
Emergency extension was achieved by dumping the hydraulic pressure. On the Arrow there is a three position lever between the seats which allows selection of Auto (normal retraction/extension and auto extension) Emergency extension (Pushed down) and Override (Locked up, Auto system is disabled). One other feature of this system is that the undercarriage will automatically extend if you get too slow and if hydraulic pressure is lost.
I can't recall if all the Piper Cherokee series had the Auto Extension function but the PA28R 180/200 definately did.
I also believe that on some aircraft the Auto extend function was locked out.
Cheers Halifax