On the valve stem seals you DON'T have to pump air pressure into the cylinder to keep the valves from falling. Just use common sense.

1. Remove the valve cover & rocker arm assemblies and keep
up with where they go. Do not mix match! My .02 cents!

2. Rotate the engine clockwise until the cylinder you are
working on has both valves CLOSED (cam lobes up for
the valve you are working on).

3. Take a long bar (long wrench will do) and lay it flat across
the top of the valve stem asjusting nut and press down( one
hand on each end of the wrench) until you are sure that the
valve "bottoms out" on the piston top. Keep in mind that the
valve spring has NOT been removed yet so you will need to
press with some force. Once you have determined that the
valve is not going to fall down into the cylinder then you can
disassemble the valve stem nuts and spring only for the
valve you are working on. You can remove the two valve
stem assemblies for that cylinder but I prefer to work on one
thing at a time. Less to lose or screw up!

4. Remove the old seal. I used needle nosed pliers. The
old seals will come off with some mild persuasion.

5. Place the protective seeve (which SHOULD come with your
new seals) over the valve stem and then place the new seal
over the stem as well and press the new seals into place.

6. Move to you next valve (rinse, lather, repeat ).

I am sure that someone has a better, more "engineered" ( no offense to any engineers out there) way of doing this but this works for me. Please note, I will add that on #3 in MY procedure you can rotate the engine SLOWLY to achieve a higher "bottom out" position. Make sure it's clockwise and NO valve springs are removed. The valve will rest on the piston top while you work on it.

Kevin