I wouldn't really know how often it happens but I imagine that if some
debris fell into the pump through the resevoir fill opening, it could become
wedged in the pressure line in such a way as to severly impede the fluid
flow. Something like some bark or leaves from an overhead tree or a little
piece of rubber or plastic that falls in when the cap is off during service
or when checking the level. I guess the actual hose lining could also
collapse causing a restriction too but I would imagine that in that case,
you would see a rupture. I would probably suspect the pump as a primary
problem but if you are like me, you want to be absolutely sure before
spending the money and finding out it was just the hose. I guess, for that
matter, you could always just take off the hose and blow compressed air
through it to find out if it is restricted.

Good luck.

Razor