Actually, it usually is in the final drive- the stuff you didn't expect to break that you listed in your post. The pump and output gear on the pump usually aren't what breaks.
I've either seen or heard of splines stripped off of axle shafts, broken gears and cracked cases. And, most of the time, it happened on a 455. Just a theory, but the torque of the Diesel, maybe?
This is a confusing valve for a lot of people. Why do I need it with a 40 Loader?
Part of understanding the significance of this valve is is to realize how torque is developed. In order to create torque, there has to be resistance. No resistance, and no torque- think of a loose nut spinning down the threaded shaft of a bolt with a wrench. The wrench is rotating the nut, but no effective amount of torque is required because there is no resistance.
The opposite is true- if that same nut is welded to the threaded shaft, it is now capable of providing an infinite amount of resistance to a force applied to it because it can't spin. Of course, we all know at some critical point the bolt will break- which is the maximum amount of torque applied before it happened.
Talking tractors, that "spinning nut" is your rear tires on ice. Very little resistance, and very little actual torque being applied to the final drive.
Put the same tractor on dry concrete, heavily weighted and up against an object or pile that won't move, and you now have a situation where the rear tires can't turn. You now have a "welded nut" scenario, tractor-wise. Under this situation, the pump can build enough pressure (torque) against the resistance (excessive traction) to break something in the rear end. The relief valve, only installed in the FORWARD hydraulic circuit, allows the pressure to blow off at a critical point just before enough torque is developed to make awful, very expensive noises from below your posterior...
In Scott's case, the amount of resistance developed was never high enough to necessitate the valve blowing off. The tires broke free first.
Running a 40 loader on a 4X5 series tractor without the relief valve installed is a really, really bad idea. If you use the loader hard enough, you will grenade the transaxle at some point.
(Message edited by dlnw98 on January 27, 2015)