The test equipment on T-1 included an air data trailing drogue that hung from the top of the T-tail. The wheel is a device to reel the drogue in and out in flight. It was this installation that inspired a fuel drogue mounted at the top of the T tail for the tanker version that no one bought.
RE the hydraulic system questions: The C-17 has FOUR independent hydraulic systems. Each of the four systems is powered by two engine driven pumps, plus an electrically powered aux pump. So three pumps for each system. In addition the two inboard systems (systems 2 and 3) have a hydraulic motor/pump interconnect. Thus system 2 can be used to pressurize system 3 and vice versa. The C-17 also has a RAT to provide hydraulic power to system 3 in an emergency. T-1 had essentially the production configuration hydraulic system.
All four hydraulic system reservoirs are accessible from inside the cargo compartment, so hydraulic reservoirs can be topped off without support equipment and can even be done inflight in the event of an emergency.