Lid not sealing on Keg

Last night I went to keg my fourth beer - the first three were pretty painless, I sanitized the keg and beverage lines and then transfered the beer, and purged O2 and pressurized for a few days @ 30psi.

This time, I was not even able to get through the sanitizing. After I put the solution in the keg, I went to pressurize it and run the sanitizer through the beverage lines. I put the CO2 @12 psi, but I heard a hissing sound. I sprayed the top of the keg with a soap/water mix, and saw that there was a pretty substantial leak around the lid.

I applied some keg lube to the O-ring (which looked to be in good condition), and tried resealing it multiple (20+) times, even reversing direction in which the lid was facing. I know this keg can seal properly, as I did a batch in it a couple of weeks ago. I have about 5 other ball-lock kegs that I can try (received all of them used from a local bar), but I want to figure out why this one won't seal anymore.

What tips do you Masters 'o Kegging suggest?

Check/do the easy stuff first. Feel the sealing surface of the keg and the lid and see if you can feel anything, then swap the o-ring with another one and try it. Try letting the lid seal without the clamp down, then clamp it after it seals. Some kegs need more pressure to seal the lid than others, maybe try lifting the keg by the clamp for the lid while it is pressurizing to help it seal.

Any chance you got the lids of two different kegs crossed? Sometimes they can be picky and a particular lid will seal better with a particular keg.

My 'racetrack' lids are the hardest to get to seal but after piddling with it they always eventually seal.

Some people say to hit it with 30PSI to seal. I've never had to, but try it.

I always try to initially seal mine with 30 psi at first. That seems to seal the lid. I release the pressure and set the psi down to 10-12 to carb the beer. Before I latch down the lid, I also lift the keg by the bail on the lid and turn my wrist left and right to make sure its seated where it should be.

+1 on the 30 pounds of initial pressure. Also, I sometimes submere the lid into very hot water, then pop it onto the keg and give it the gas. The hot water softens the O-ring. Good Luck...

Some of mine I even have to crank up to 45 PSI.

Won't hurt anything.

Temperature?

When my lid and O ring are coming out of a soak in HOT sanitizer, the ring is much softer and easier to seal than if it is coming out of the cold fridge.

Also, sometimes it's just time for a new ring. They seem to get less supple as they get older (*sigh* if it were not so...)

William's Brewing sells a softer o-ring for sealing difficult lids. Not cheap ($4.90), but if it solves the problem...

Williams Brewing keg Lid O-ring

I did a writeup about this issue quite a while ago. Please take a look:
Keg Lid Geometry & Adjustment

You can click on the images for a larger picture.

Hope it helps.