My Gravely was bought about 3 years ago with NO brakes at all. I very quickly added brakes for steering the beast. Otherwise, man handling it on hills was neigh onto impossible for me.
I would think that HST tractors hardly need brakes, mainly just for parking them with the engine running. I only use the Gravely brakes for steering it and for parking it. For slowdown or stopping, I pull the fwd/rev lever back a little bit to reverse.
No front brakes on it either, no front wheels!
Ralph