Yes, the case drain on the injection pump has a check valve in it. It is called the case pressure regulator as case pressure influences timing. More pressure means less advance, lower pressure advances timing. There is a solenoid which unloads the check valve spring, causing case pressure to drop about 5 PSI (cracking pressure of the valve) which is how the cold advance solenoid works.
Transfer pressure or charge pressure is speed dependent, a circuit pressurizes the timing advance piston using this pressure. There is also a low speed cam on the right (passenger) side of the pump which is actuated by the right side throttle shaft.
Back to original topic..a check valve in the return circuit downstream of the injection pump will only mask problems and retard timing. If you are having an air leak, find it and fix it. Injection pumps need a constant flow back to tank to keep the injection pump cool. They are constantly stirring the fuel which generates heat.
