Jim, I've thought about this off and on for a couple of hours. The only thing that I think might cause this is air in your hydraulic system. The first thing I would check is the level of fluid in the rear axle. If it is low, the pump might pump air and fluid into the cylinder. When releasing the cylinder on a light load, it might not go down.
Another possibility is the type of oil in your rear axle. The fluid should be 30W non detergent. Some use regular hydraulic fluid. Motor oil that contains detergent may foam in the system giving the same symptoms as air in the system.
It is messy but, if you aren't able to find anything else wrong, drain all the fluid from the rear axle (the rear axle and transmission share the same reservoir with the hydraulics). It contains 6 quarts of oil. Replace it with non detergent 30W oil. About half of the auto stores in my area carry non detergent oil in 30W. If you can't find it, use a good grade of hydraulic fluid which is more like 20W. If you have leaks, they will be worse with the hydraulic oil.