Okay, here is why this happens. When you backdrag, you are putting steady pressure on the rod pushing the piston into the cylinder. The constant fluid pressure from the piston side is bypassing the valve spool, and going back to the tank. The style valve in use does a poor job of holding constant pressure in the neutral position. Just look at how your stabilizer and backhoe cylinders drop so fast in the neutral position. Since when the piston retracts it pushes more fluid back than the rod side requires, air is introduced into the rod side of the cylinder. The rod seals do a great job of keeping the fluid in, but a very poor job of keeping air out. Air also easily bypasses the piston seal to get into that side of the cylinder also. Irregardless, with air in the cylinder, you get bounce.
Now, how do you prevent this? If I owned any of the tractors you guys do that have cylinder bleed down issues, I would complain to the dealer and the manufacturer. Cylinder drift overnight is not normal, unless the dealers and manufacturers convince you that it is in order for them to use cheap valves. I have seen dozens of similar applications that have virtually no cylinder drift.
