Today I troubleshot and fixed my non-working farm diesel pump. 30 years ago or so Daddy bought 3 pumps from the airport when they got new ones. We keep on-road taxed diesel in the in ground tank the pump is attached to. I have noticed the pressure at the nozzle dropping over the last couple months and attributed it to infrequent use. The off-road non-taxed is used a lot more. Then it got to where only a trickle would come out. I just had 500 gallons delivered so low in the tank wasn't it either. Replaced the slipping pump v-belt and that wasn't it either.
I pulled the primer plug out of the pump and took a small diesel shower
Obviously plenty of pressure in the pump. I think collapsed or plugged hose. Hose off, pump on dribble. Hose back on. Then
I noticed the gallons meter wasn't turning....and a shaft from it goes in the top of the pump 
Turns out the pump has had a broken window for years letting rain into the mechanism. Sprayed liberally with PB Blaster and worked the shafts with vise grips and it freed up. Cranked the pump and it pumps like a new one, with the gallons meters turning... Whoda thunk it???