problems with QB78 in 177 -massive leaking

just bought a second hand and first time i changed carts all the gas comes out the breech and barrel and after 30-40 sec gun is empty - now wasted 8 carts and already loosing interest (again) in CO2. Just bad luck and I am not blaming the seller . I have in detailed followed all instructions I am am 100 percent convinced that the gun needs resealing (i am thinking all seals incl valve) . I have resealed my benji pumper without problems but never a co2 gun . I have read many of the older posts here on the QB78 and heard of Melick and Archer and so on - seen Archer on youtube but really not much on the who;e reseal unless you buy the $27 repair book ? . I am really thinking making it into a bulk co2 (but not too bulky) or pcp but then again dont want to spend too much. Dont have pcp gear anymore though. What would you do ? sorry for my rant but I am starting to think I am done with co2 even though this one was really for my kid to use . Thanks !