Ford 429 hydraulic lifter question?

When I took my rocker arms off my head, to put seal on. I went to tighten down the rocker arm stud. It has no compression now. the lifter seems like it is holding the valve open. But I have compression when it is not tightened down. What could make the lifter stay pumped up or what could be holding the valve open? The valve is not bent & the rockerarm is fine.