I have had no end of problems with my less-than-budget pressure barrels in the past. A king keg and Hambleton Bard Beer Sphere.
After much searching around on here and trying to tighten everything, I found that the seal around the lid was the culprit. I only found this out by nearly filling both barrels with water and pressurising up with CO2, the submerging in a plastic dustbin. A slow leak (bubble) was visible around the lid after several minutes submersion.
To solve it, I have (following advice on here) employed two tactics:
1) I used double-sided carpet tap and stuck some fine wet and dry paper to a very flat, glazed bathroom tile (30p for the tile). I then carefully sanded the lip on the very top of the barrel until it was perfectly flat for a minute or so. It already looked flat to me, and I couldn't feel any lips/bumps etc, but this bit had been proved to be the area of the leak. After a thorough cleaning, I put the lid back on with a very thin smear of vaseline and to my great surprise, the barrel now seals with a very easy one-handed tightening. No more of this wringing the heck out of it to close it.
2) To keep and eye on the thing, I have fitted a 1 bar/15 psi pressure gauge in the lid. This has verified that I do not loose pressure, and it great for keeping an eye on the serving pressure. I like to serve at around 2-5psi, and top up to 7-10psi for storage. Serving above 5psi tends to cause gushing
Good luck.