so what you are saying is when you are lowering , with mast collapsed only with carriage/lift bracket in the first stage, IE: free-lift mode and you stop the lowering, the carriage/lift bracket will keep traveling down on its own for a few more inches before it stops? This is what it sounds like your saying to me.
If it is i had some trucks at a customer that started doing this and it did wind up being the safety valve in the base of the cylinder rod. The only way your going to know for sure is to remove the rod and take a look.
When i did this on those trucks the safety valve was either stuck or completely blown out and in pieces hammered all to pieces at the bottom. Had to get a magnet and fish out all the pieces, clean out the lines etc.
We found out it was due to a faulty snap ring breaking or coming out that holds the safety valve in place.