Well first, I don't even know where the PDI system is located. See i dont have the owners manual to help me on this one so i really need your guy's help. Maybe there is an online site that can help?
okay, im guessing you are talking about the piston head right? Well first you take off the slide (pop out to take down lever and just slide off the slide). Then, you take out the recoil spring and spring guide Then you take out the barrel assembly Now, you look at the very back of the slide and you will see a small screw; Unscrew it and set it aside (im not sure if this is necessary because i am telling you this from memory, though it doesnt hurt to take it out anyway) Now you can slide out piston assembly in the very back of the slide (this will be slightly difficult to do as it is normally packed in there tight) After it is out, you take out the spring on it and slide out the front section of the whole assembly Now, if you look inside the main body, you should see the piston head with a screw in it; just take out hte screw and put in the new piston head and reassemble everything.
I'm pretty sure thats all of it though i may be missing a step since i dont have the gun anymore and am telling you this from what i remember.