If they are the originals I'd seriously consider a new/recon pair, mine were around £50 each when I did mine.
If you want to try and pump the piston out, simply keep the old pads in the good side, and insert a spanner between disc and good piston to take up space and pump.
Don't be at all suprised if it doesn't work though, my set had 2 out of 8 pistons working, managed to free off another 4 after about 10 hours swearing etc, had to machine out the other two to paper thin then crush in, just not worth the hassle, I then discovered 2 stripped threads for the bleed nipples.
WD40 won't ruin discs simply clean off, diesel fuel works vastly better than WD40 though!!
tabetha