Those pumps are pretty bulletproof. The drive gear is held from free spinning by a woodruff key. I have one on a 3600. For those unfamiliar with them, the pump, reservoir and filter, as well as 2 valves are all in one self contained unit. When you open the fill cap, with the engine running, you can see the fluid streaming out of the filter if everything is running right.
Here's what the Ford 4630 pump and other steering parts should look like:
Even if the piston seals on the steering cylinder are blown, the size of the piston itself will have offer some resistance to flow in the bore.
There is also a regulator valve in the drive motor--the part up under the dash below the steering wheel, and the rotating drive motor parts are held to the steering wheel shaft with a drive pin.
The 2 sudden onset appearing problems that come from this are usually :
1. The drive pin (#6)that holds the shaft is sheared.
2. The regulating valve spring (16) is broken or the valve (20) is stuck open.