AKAIK, it should be located on the lift pump.
If you have a horizontal fuel feed line it is sandwiched between the plunger style primer pump and lift pump on the DS of the injection pump.
If you have a vertical fuel feed line, it is under the nipple for the fuel inlet on the lift pump.
It prevents drain-back of the fuel when the engine is not running

There is also a pressure release valve (fuel overflow valve) on the engine side of the injection pump.
It is located in the banjo bolt for the clear hard fuel return line, and is serviceable unit comprised of a spring and ball bearing.
It helps maintain fuel pressure in the low pressure fuel circuit of the injection pump.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022