I have a GT225 that's 10 years old with only 15 HP and there is not a hill on my property that it will not only mow going up, but it will also do it pulling a loaded cart, and I have some pretty steep hills.

I remember seeing a GT225 once at the local dealers (it was in for repair) that was set up with for mowing steep hills. It had the rear wheels reversed, had wheel weights, and tire chains. It was equipped with a 48" deck.

And so I have trouble understanding why the X520 wouldn't go up a small hill, either forward or backward. I would suspect a loose drive belt.

Also when going down a steep hill on a tractor, either big or little, you never take it out of gear; you let the gearing hold it back. On a hydrostatic tractor pushing in the brake takes it out of gear, and the brakes were not made to hold back the tractor going down a steep hill.

I use a Stiga Titan, which has independent hydraulic four wheel drive to each wheel. I've never known it to lose grip yet, even in the wettest conditions, and even on the tightest turns it doesn't scrunch the turf because it has articulated steering.

OK, the Titan is a commercial grade machine, but Stiga and Husqvarna do make cheaper machines for the consumer market that also have four wheel drive and articulated steering.

I wanted a machine which could help me level the low spots with a plow blade and wood chips (or clean fill) as I get them. I decided to get a used late 80's Case/Ingersoll 444 (with the installed holding valve). Im still saving/looking for a plow blade.

I have been told that my C/I's holding valve is kinda like a hydraulic brake - which I dont completely understand yet... But I'm told its used to descend "safely" down very steep inclines - which my property has plenty of. Im told - this hydraulic holding valve is frequently installed on older cases tractors (22x/44x) which didnt have them - and are working on properties hilly/mountainous like mine.

The tractor is completely hydraulic to the back axles - kinda like a bulldozer (not to be confused with hydrostatic on many other tractors).

I can say that there are zero issues going down any steep inclines at my place. I dont have to use the brakes or anything - just relax and let the valving do the hard work. It has an interesting sound when dealing with these inclines - sort of a high pressure squishing sound.

In addition with the 16 inch rims and very large tires it can just as easily climb those same inclines too. (I have not used chains - yet).

One interesting point - the fenders for my model are rather large and eventually become the foot rests to the tractor (the stamped steel is very heavy gauge). Anyway there is an angled lip on the tip of the foot rest (where your toes would be), which looks a bit oddly angled upwards - too steep to be of any use...

Well - when your going down a very steep incline those angled tips are what allow you to sort of stand onto them to support my butt from sliding down off the seat - which would ultimately be a crotch slapper into the steering column (or worse - my feet sliding into the front wheels! )

Another nice feature is that reverse on this tractor is as strong/forceful as forward is, and I dont have to rev the motor out to climb or descend the steep hills (which I have tried in reverse!).

Pictures of these foot rest can be seen here.
http://s247.photobucket.com/albums/gg160/njdpo/ing444/

Ive had the machine now for about 2 months - overall I'm very pleased with the abilities of this very well built machine.

Good luck with your search.

http://www.gravely.com/Commercial/outfronts/outfront/320hd/Pages/default.aspx

It steers by the rear wheel and has a very wide stance. You will be able to go around hills as well as up and down. Not a zero turn (although it nearly is). It's like the old Yazoo master mower that I've been using for the past 5 years or so.