Thanks DaveG, I think you solved that issue for me. These are likely very old. They seem to work, but I was hoping the common problem they share was due to the packing. So ... going a bit off topic, most of these brushes have a bubbling issue with the bottles. I found that if I applied some thick old to the threads of the aircap/head, and really cranked with a wrench, I could stop this. But I don't think I should need to wrench to get a good seal. I replaced all the O-rings. My guess is that these are really old, and I should consider replacing the heads? Also, they seem to have some issues at 50psi sucking paint up the bottle. The bottles are new, and cleaning fluid will flow just fine. I've tried thinning the paint, but this does not seem to matter sometimes. It seems the slightest issue and the paint splutters. Since this is common to many (4 out of 10) guns, I am trying to find out why. So ... I guess I'll google the common reasons for spluttering paint. Thanks all for your help so far!