I am currently measuring for pushrod length on an old 350 that I am rebuilding. I am reusing the hydraulic flat tappet lifter that were in it from a previous rebuild and noticed that the exhaust lifter is very soft and compresses very easy when trying to measure the push rod length. just screwing down the rocker nut compresses the lifter. all the other lifters seem fine except this one. I'm thinking maybe I have to fill it up with oil ? but I've never done this to a lifter before?