The diaphragm is made from metal but it does have rubber seals. It usually will crack after many miles with low lift pump pressure because of the constant pounding it will get when the VP high pressure solenoid valve releases the pressure to end the injection event The released fuel flows back to the interior of the VP and extends the diaphragm. This pulse of fuel also causes the infamous hammering of fuel pressure gages as the diaphragm extends pushing the lift pump fuel back toward the lift pump. Didn't Bosch make a wonderful injection pump?

 

Precisely.

But regarding your feelings toward the VP, I'm not one to categorize the VP as garbage. I think its a great pump that does a great job. Other injection pumps are also prone to failure as well..........just not at the level of the VP. And some of the most fuel efficient Dodge Cummins trucks are VP engines. I just feel that its a shame that Cummins, Dodge, and Bosch obviously had a laps of communication and/or possibly an unfortunate lack of R&D for the ISB application. It seemed that because they all knew the VP was simply a temporary fix until the Common Rail could be finished, no one cared. Maybe they also didn't realize how popular it would become. Honestly I really feel that the latest "theory" regarding the fuel pressure staying above the overflow valves 14-16 psi, is going to be right. Now if we could only get Bosch to give us some computers that have lead solder then we might have an respectably reliable injection pump.