Wilf,
It is just a thought of something to look at. I am not 100% sure of the internal design of a hydraulic lifter rather a basic understanding of the function. I know that there is a small
oil galley in the lifter that holds
oil and that there is a "feed" hole in the side of the lifter from the block, and also a hole in the "cup" where the pushrod seats, if this has somehow become partially plugged it could cause a pump-up condition. If I remember correctly there is a valve style action (of some sort) inside the lifter that controls
oil flow through the lifter. If this is defective it could allow excessive pressure buildup.
Another thought would be that maybe one or more pushrods are not hollow core (or maybe plugged) not allowing oil to pass up through it to lube the rocker arms/valve guides which in turn would possibly cause a pump-up condition. Anyway about it these are just thoughts for things to look at.
John