My other truck is an '86 F350 with a 460. The valve stem seals are pretty much shot, and the truck smokes real bad on start up and high vacuum.

Today I had planned on replacing the seals, but when I hooked up the air adapter to the head, the engine wouldn't hold but maybe 30 psi. The valves were definitely shut, and the truck was in gear with the parking brake set.

That seems to be a pretty good indication that the engine needs more than just valve seals, and I'm not going to risk dropping a valve on an engine that will need to be pulled apart anyway, especially when I don't have a full rebuild budgeted right now. I'm just going to put the valve covers back on and keep an extra quart or two of oil in it for the time being.

Any thoughts?

Did you notice where the air was going to? was it coming out the exhaust or intake?

Sometimes you can take the rocker arms off and smack the valve slightly with a rubber mallet to open the valve quickly with air pressure applied it may seal better, allowing you to do the seals.

A chunk of carbon may be holding one of the valves up.

Worth a shot.

Did you try more than one cylinder? if this is happening on more than one cylinder and you do not hear air escaping from the intake or the exhaust you may just have a faulty gauge or compressor.

__________________
2002 Ford Ranger 2 Dr Standard cab 2.3 5 SPD 7.5 373 gears

I'll look into that next time I have a chance.

I couldn't tell exactly where the air was going, but it sounded like it was going out through the somewhere in the engine

Could have worn out piston rings.