Does your directional control valve have cables to operate or is it all mechanical linkage? From what you are describing it sounds like the valve may not be centering properly all the time. This can be from moisture in the control linkage, moisture in the spring caps on the end of the valve spools etc. If the valve is not leaking and the cylinders are full of fluid the loader cannot leak down fully even with bad seals on the cylinders.
mechanical i would imagine. im just curious as to what would cause it to happen. it only happens every once in a great while. usually when i set the bucket curled under over night, like when the oil goes back 'into' the tractor it sucks air in the system