You might try replacing the filter near the valve stack outside the cab or the filter in the tank. Might also be abad o-ring on the relief valve assembly. If you could put a gauge on it you narrow it down.Hope this helps a little.
which 1480 do you haveIJ 80 or earlier (manual hydraulics) or 81 or newer (electro hydraulics)IJ We had both. It took a new relief valve on the older one to solve this problem (i'm assuming your talking about slow header lift). The electro took a valve cleaning a time or two.