We only use the 4020 hydraulic systems to raise and lower the mower bars on one tractor, and to raise and lower the bale bed and loading arm with the other. In other words, the hydraulics aren't being worked full time. Seems like there are always drips and leaks, but even the Space Shuttles suffer those problems.
When Dad first bought one of these 4020's new, in 1968, a few years later he used it to rake with a Vermeer hydraulic-driven rake. At that time, with the hydraulics in constant use, it seems like it would take a new hydraulic motor about once per year. At the time, $200 seemed to cover the expense. That much money probably wouldn't anywhere near be enough to solve the problem today.
Don't know what to tell you, TimH, except that the hydraulics might be a weak spot on 4020's. They are good old tractors otherwise.