OK, so my silca pump has a rather bulky head that I thread on to the tube stem, then attach the hose from the pump and get to pumping.

The problem is that the connection leaks air (the QD connection), so the pressure doesn't hold and I'm never exactly sure what pressure I'm getting in the tire. It's not a huge problem, obviously, but it's a small annoyance. Is there some way to keep the connection from leaking? It has leaked since day 1. I thought I'd check before putting a little grease on the o-ring in the head.

I think it's an air disconnect issue more than a silca specific issue - this is what it looks like:

Never had a problem with the Bell presta head on my Silca Track pump I got 30 years ago.

only air escape is the hose is full of air when I take off the pump head,

as there is a check valve in the tube valve stem and the base of the pump.

Grease (use silicone grease if you have it) the O-ring to improve the seal.

Use silicone grease on O-rings; petroleum-based grease can degrade some types of O-ring.

Or maybe consider a different head so you don't need a quick-connect; I just got a Lezyne ABS2 head for one of my pumps. It fits on Presta or Schrader valves without adapters and goes on and off easily without leaking appreciable air. Lezyne - Engineered Design - Products - Digital Pumps - Accessories - ABS2 Chuck HP

I'll try the grease approach first since it's cheapest. Thanks for the silicon tip.

Okay, I have that same pump head and it just sucks. Incredibly slow to use. I found an old brass head in a junk box at a bike shop, and keep meaning to install that instead. For schraeder valves, I just use the air conpressor.

Well silicon grease didn't seem to improve anything and of course, it's still a PITB to use. The only real advantage seems to be that I can use it on very short valve stems, since it screws on but I'm damn sick of it.

Off to the replacement head store.

The Lezyne I recommended screws on and should work with any stem which protrudes enough for it to catch. It screws on so can be easily used even on a deflated tube since there is no pushing required. It even has a bleeder valve. They have a high pressure and a high volume version; it looks like the difference is in the hose size accommodated.

I emailed Lezyne to see what the difference and the hose diameter accommodated. My old performance pump needs a new head too and has thinner tubing. The Silca tubing is like aircraft hydraulic tubing, it's stupid thick.