CAV Injector Pump on a Ford 4000, 3 cyl?

Having problem with pump getting fuel to the injectors.
New pump put on 6 months ago, cranked and ran all day. a week later will not start no fuel at the injectors. Sent pump back to dealer, they sent me another one, Put it on, cranked right up ran all day. Next day same problem. Sent it to a rebuild shop,they said plunger stuck had it rebuilt for $400.00. Put it on ran for a day. Has new tank, new lines, new injector and filters. What's wrong here? Will not crank again today. Three pumps in a years time