I never had much luck with the hydraulic method using a tight fitting rod and a hammer.
It always seemed to glance off at a slight angle and spill out the mouth, without freeing the cap.
But I tried a variation on the water method, using the correct reloading die and shell holder to support the case.
--REMOVE THE DECAPPING PIN FIRST. This is kind of important...
--lube and resize the case as usual, except the primer remains.
--Fill case with water, and slam it hard up into the sizing die again. THE CASE MUST STILL BE LUBED! This is also kind of important...
--Expect water to flood out the bottom. My press has a hose to collect spent primers and direct them into a bucket.
--Remember to wipe down and oil your press after.
I get about 100% success at the first thump this way.