I have several kegs like that. I keep a spray bottle of star-san handy, and always spray a keg when I fill a new one and give it some pressure. I spray the lid, the posts, etc, and look for leaks.
One thing that really helps is using keg lube on the o-ring. And then "wiggling" the lid around when you close up the keg to find the sweet spot to sit it. I have one keg that always leaks, so I put a dime under each "foot" and that fixed it!
Broken Leg Brewery
Giving beer a leg to stand on since 2006