I don't think the injection pump has a check valve, that is built into the lift pump as the in and out valves.
So, why would you be looseing a prime? The check valves could be dirty and bleeding back when the engine is at rest, but you probalby have Racor filters and they also have a check valve so the chances of three check valves failing is pretty remote.
You might also have an air leak in the suction side of the fuel system. This is very common and you can't see the leak casue it's air leaking into the fuel system rather than fuel leaking out.
You can find this leak by pulling the fuel line right at the tank and hooking up a small electrci fuel pump. Let the fuel pump build up presure. After the sytem sits overnight inspect the complete fuel system. You'll find the air leak as a diesel drip.
This the common system of finding diesel air leaks, and mostl likely this will be the problem.
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You can use an air compressior but not over 7psi. This is very important, you need to keep the presure down.
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