Are you talking about takedown pin/lever from a PX4 subcompact? If so, carefully inspect where the takedown pin goes(i.e. the hole from which you pulled it out of). If you notice, a spring is angled in the whole and you must, or should reinsert the pin's cutout at that angle, about a 1/4 turn up or so and push it in. To field strip, rack the slide to the rear, rotate the takedown lever down, gently pull out 2 clicks, but do not take the takedown pin all the way from the frame and take off the slide. Assemble in the same manner but reverse. You kinda have me confused over "lock pin" and "slide pin"?