Two hour window. That’s what I give myself as a max time amount my impact is empty of all air. It’s been my experience, if in any task that is being done to my impacts take longer than two hours with the gun completely empty of air, I better plan on leaks as soon as I fill back up with air. Typically the regulator areas are the first to leak, and if not there, a leak will be then tied into the power plenum area. Ive been lucky the last couple of times as the o rings in these two areas were all replaced not too long ago, so when I went over two hours the leaks came from the gages, so I replaced the x rings on those. Easy fix.
These o rings are just taxed when doing their job in holding in high pressure air. They take on a whole new shape of their own by being stretched and pushed against one side in the groove they sit in. Some areas of the o ring get flattened. In the time the gun is empty of air, the o rings are now slowly falling into a relaxed state back in their grooves but because they’ve developed flat spots and have been stretched, the minute the gun is refilled with air the flat spot that used to be on an o rings side may now be on top, and that’s where the leaks come from. That’s just my theory on things.
Just yesterday I drained down one of my impacts to replace the rear block. I made sure I had everything ready at the bench, all parts, tools, and lubes. Had the block replaced and air’d back up within an hour and twenty minutes. No leaks
Lastly- going hog wild on silicone lube on o rings will cause leaks, also, especially on regulator o rings within an air tube. The excess lube will cause the o ring to push out of its groove and roll to the side. Just swapping out the o ring for new probably won’t fix it, either. You must first clean the air tube dry by removing all lube with acetone. Ive had to resort to lightly scuffing the area the regulator rests at with 1000 grit wet/dry sandpaper, wipe clean with acetone, and now the new o ring has an area to “grip” against and not roll off its groove.