What Jack said!!! :thumbsup::thumbsup:
Going down a fairly steep hill on our property, I keep my left foot on the reverse treadle out of habit, but I've only had maybe a couple occasions in 14 years where I've had to hit the reverse rather than just letting up on the forward when I had a heavy load in the bucket. Most of the time, letting off the forward pedal too fast will jerk you to a stop uncomfortably fast. The hydraulics are that responsive.
Yes, the parking brake is just that... for parking. It won't stop you. If you engage it while you're moving you'll hear a loud tapping noise as the pins (one on each front wheel motor) slip past all of the holes in the wheel hub. It will engage if you slow the tractor with the pedals, but you'll be almost stopped before that happens.
The only thing I'd keep in the back of your mind is that a hydraulic hose could catastrophically break. Then you may free-wheel quickly I suppose. And if you're pointed towards a body of water, a steep drop-off, a tree, a house, a highway, etc... you'd better have a plan. But similar things can happen with conventional tractors. Prime example is a heavily loaded bucket, going down a hill forward, two-wheel drive, and the front load is heavy enough to reduce traction to the rear tires, and tractors don't have brakes on the front axles.... down the hill uncontrollably you go.
So you drop the bucket and try to tilt it forward so it stabs the ground and ejects you over the steering wheel!
Now if you have a mower out front instead of a bucket, and you try that, the mower has wheels, so you'll keep rolling down that hill! You could try to turn and hope you don't tip over. If you're buckled in keep your hands on the wheel and don't reach out or you'll break your arm on the ground. Just hang on and stay inside the ROPS and canopy. Chances are good you'll just land on your side if you tip. I've heard of a few PTs on their side, but have never heard of one going over. That ROPS and canopy used with the seat belt is your best bet.