Hi Bobby. I have been working on Mf injecjection pumps for more than 40 years. DPA pumps, in my opinion, the best ever made. However the most important and obvious reason for liftime service of any injection pump is clean fuel.You say you have replaced a rusty fuel tank. Water in fuel is fatal for any pump and it normally causes a seized hydraulic head in the case of a DPA pump. To determine if it is the case, remove the transfer pump endplate at the back of the pump and then turn the engine or the pump itself if it is already been removed from the engine. If the transfer pump rotor does not turn, you have a seized head. If it does turn, then you will have stuck pump plungers. To rectify this the pump will have to be stripped an the plungers loosened. This will have to be done by a pump mechanic.
To come back to DPA pums. I have, through the years, done many conversions where I raplace any mechanical injection pump with a DPA. The majority were replacements of the Bosch VA pumps on the German made International tractors with the Neuss engines. On two occasions I have replaced Bosch VE pumps on the 3 liter Tata ldv's with DPA's.
Paarl, Western Cape , South Africa