Can an over-crimped PEX pinch clamp cause a slow air leak?

That rate of drop is a legitimate leak, but don't drive yourself crazy cutting out every joint just yet. First thing you need to do is **isolate the gauge** and your fill point. I've spent hours chasing a leak on a rough-in only to find out the Schrader valve I used to pump it up was hissing just enough that soap bubbles wouldn't even form. Put a cap on the end of your test rig or spray the hell out of the gauge threads and the valve stem. If the leak is there, you're chasing a ghost in the walls for nothing.

If the test rig is solid, look closer at that brass elbow. Over-crimping with stainless steel pinch clamps can absolutely cause issues. If the tool wasn't calibrated or the teeth were misaligned, it can actualy score the PEX or slightly deform the brass barb underneath. When that happens, air can snake its way through the serrations on the barb. It’s a tiny path, which explains the slow drop. If it looks like it's digging in way too deep compared to the others, it's worth cutting it out and replacing it just to be sure. Since you aren't seeing bubbles, your soap mix might be too thin. Straight dish soap and water often runs off before a tiny leak can blow a bubble. Get some dedicated **leak detector fluid**—the thick, snotty stuff—and glob it on. If it's a super slow leak, you might have to leave the soap on there for a few minutes and look for a head of tiny foam rather than a single big bubble. If the air is blowing the soap away before it can form a bubble, you'll never see it without the thick stuff. You also need to isolate the hot and cold lines if they aren't tied togehter. If you've got them looped at the fixtures, break the loop and test each side independently. It’ll cut your search area in half immediatly. Also, check the plastic manifolds if they used them; sometimes those have hairline cracks from the factory or from someone over-tightening a connection. Don't write off temperature entirely, but a 1 psi drop every ten minutes in a steady-temp apartment is almost never just the air cooling down. You've got a hole somewhere. Check the tub spouts and any stub-outs where someone might have accidentally nicked the pipe with a drywall saw or a screw. Those are the ones that really bite you beacuse they're often hidden just enough to be annoying.

I'm still learning the ropes, but I saw a guy on another job struggle with a similar drop becuase of the manifold. Are there any plastic shut-off valves already installed on this run? I wonder if a tiny bit of debris or a bad o-ring in one of those could be letting air past the seat, even if the handle is turned off. Also, is it possible for the gauge itself to be a dud? My boss says some of the cheap ones from the big box stores can leak right thru the face if they've been dropped or kicked around in a tool bag too much.

Check your shower valves if they’re alredy roughed in. I once spent a whole afternoon hunting a ghost leak that turned out to be a tiny bit of grit in a pressure-balancing spool. Air will whistle right past a cartridge seal that woud hold 80 psi of water without breaking a sweat. Pull the cartridges and plug the housings to see if the drop stops. Also, check any transition adapters. I’ve seen brass fittings with tiny casting flaws in the threads that only show up under an air test. If that suspect elbow looks wonky, just swap it; it’s cheaper than the labor you're burning thru.

Check the orientation of those pinch clamps. If the 'ear' isn't centered right or the ring is sitting slightly crooked, it'll leak every time. Also, take a closer look at that elbow near the metal stud. If the PEX is pulled tight against that metal, it coud be putting enough side-load on the fitting to distort the seal just enough to let air past. I'd also wipe the pipe down and look for tiny nicks. I once chased a leak for half a day that ended up being a shallow score mark from a utility knife right where the clamp was sitting. Air finds those little channels way faster than water ever will.

Did you use a small portable compressor to fill the lines? Those things get the air surprisingly hot. If that warm air is hitting the cooler ambient temp in the walls or a cold slab, you’ll see a steady drop for the first hour as it stabilizes. I've seen people chase 'leaks' for half a day when it was just thermal contraction. Also, check the exact spacing of that pinch clamp. If it's sitting too close to the end of the PEX or acutally overlapping the shoulder of the brass fitting, it won't ever seat right. I usally aim for 1/8" to 1/4" from the cut end. If it's riding the flare of the barb, it’s toast.