Tractor Repair: Injectors Wont Fire, injector nozzles, perkins diesel

Question
I have a MF 35 with a Perkins diesel. Last week, it just quit. I disconnected a fuel line going to an injector and cranked the engine. Fuel spat out. (No bubbles, no froth.) I pulled an injector, turned it upside down, reconnected the line, and cranked the engine. No spray from the injectors. I brought all three injectors to a shop, they tested them, (in front of me,) and all the injectots tested fine. I put them back on, the tractor started right up, ran for about an hour, then quit. I've repeated all above procedures, with the same results. Fuel goes everywhere it should, except out the injector nozzles.
I should add that the repair guys warned that if the return lines were blocked, it would prevent the pump from building adaquate pressure. My return lines were fine, now kinda extra clean since I checked them.
There's gotta be something really obvious right in front of me ....

Answer
Hello,

  If you can blow through all the return lines, then the problem may be in the injection pump.  Just because there is fuel getting to the injectors, doesn't mean that there is enough pressure there to make the injectors fire.  The injection pump has to supply at least about 3500 psi to make the injectors fire.  I would recommend getting the injection pump tested.

                         Arnie