The Predom is not particularly difficult to work on but you will need good screwdrivers, a punch (maybe), and patience. I took mine apart for cleaning / lubing several months ago, so this is from memory.
Remove the grips; they tilt out from the top (held in by a tab at the bottom.)
Remove the threaded spring cover on the bottom of the grip; I had to use vice grips and a leather belt for padding to get it started. There is some spring preload - probably 10 pounds or so. No compressor is needed, just a little care. There should be two springs present.
Remove the stamped sheetmetal plate that was held in place by the spring cover.
Remove the barrel - there is a small lock screw on the right side that must be removed first; the main pivot screw can then be removed.
With the pivot bolt removed, rotate the barrel down; this will push the cocking lever down. Rotate the lever down so the little part that pushes the piston down is pointing upwards. It is then possible to remove the cocking lever from the bottom of the grip. The piston will not come out without doing this.
Now for the first annoying part - removing the piston. The sear that holds the piston in the cocked position makes getting the piston out a pain. I finally gave up and removed the pin that holds the sear lever in place. Reassembly of that is difficult because of the spring behind it. It may be possible to get the piston out with the sear in place but you would need to find a way to get the sear out of the way so the piston head can pass. Even with the sear out the piston seems to hang up on something and takes a bit of fiddling to remove. Maybe I am just not smart enough.
Once the piston is out you can clean/lube/etc. You can see the wear point on the lower, back part of the piston. I just lubed the areas that seemed to need it - parts of the piston, the spring ends, pivot screw, cocking lever tabs, etc.
Reassembly is mostly the reverse. If you remove the sear, note where the spring is as you are working. I think it was also necessary to hold down the trigger or a lever of the trigger mechanism to get the sear lever back into place. It is a pain, but just takes some patience. It must be done after the piston is back in place.
The spring cover will need to be more that hand tight or it will start to unscrew during firing.
If your mainsprings are worn out, JG Airguns has replacements for the Walther LP53 that work fine, although they may need to be shortened a coil or so. There is also some info on anotherairgunblog, but nothing on disassembly.
Paul in Liberty County