Hmm. Sounds similar to what mine is up to. It went for a period of not being used or oiled for about 3 or 4 years. First compress seems fine, subsequent ones seem to cause a pressure lossalmost like blow off valve acting prematurely - but it’s more like very little resistance rather that a pfft at the end of the stroke. But as said first stroke ok, so maybe main seal just bypassing when pressure starts building. Yet to take it apart. Had been hoping reoiling, leaving it, gentle back and forth partial stroke would help…but nope…