The valve seals can be replaced with the cylinder head on or off the engine. But great care and skill must be used to perform this procedure with the cylinder head on the engine. With the cylinder head off the engine, compress the valve spring using tool J26513 or equivalent, remove the keeper, spring and seal. All parts must be returned to their original locations.
If the cylinder head is on the vehicle, 60 psi or more of compressed air must be used. The procedure will require bringing the piston of the cylinder to be service up to top dead center on compression stoke. Then apply compressed air through the spark plug hole using a compressor gauge fitting. The valve spring and seal can now be removed.
If the air pressure is too low or drops during this procedure the valve will fall into the engine, requiring complete disassembly of the cylinder head assembly. Also if the valve is pressed downward at all, pressure will be lost and the valve will fall in.