[406 Q-ship]

The valve seals will not cause a pressure leak from the cylinder, just for oil to leak down the valve into the cylinder. Unless you have one weak a$$ compressor it will keep the valves up even in a engine with poor valve seats (burned valves not with standing).

I have always done the blowing in air to the cylinder when changing springs and valves seals I only do it with one change over the normal. There are going to be people who say it will not work but it does just fine. Remove the pressure completely off both the intake and exhaust rockers and then apply the pressure into the cylinder, the engine will rotate putting the cylinder at BDC, while holding the valve closed (there is better ring seal at the bottom of the cylinder too). Change the springs or valve seals and reinstall the valve retainers, readjust the rockers and your done. This eliminates the need to get the engine exactly on TDC.