Stuart is right about the way to loosen them. On an engine where the valve can hit the piston when wacked it is better to use the air fitting in the spark plug hole to keep the valve closed. with the rockers off the air pressure will push the piston to the bottom and out of harms way. In the 60's we did all the warranty work for Olds and Ford in the Dallas-Ft Worth region which amounted to around 105 SETS of heads a day. 90% of the retainers and keepers were removed by the big brass hammer and deep socket method; didn't have time to use even our air powered spring compressors. You cannot do this with the block surface of the head on the bench or it will bend valves when they strike the bench. Same with a piston on TDC on some engines.

Evan