oil filler cap

If you decide to change valve stem seals, it is not necessary to pressurize the cylinders. You remove a spark plug, feed white, cloth-type clothesline into the cylinder until you have the cylinder pretty well full of line.
Then turn the crank by hand, wrench, or fan, so the piston comes up and squarshes all that clothesline into a wad at the top, holding the valves in their seats. I once watched a man who used the pressurized cylinder method and he was having a hard time getting the retainers to let go. He had to do
some banging, and the spring compressor let go. When it did, the valve dropped right down to where there was no chance of getting it out.
He had to pull the head, and it sort of ruined his weekend, if you know what I mean