Check valves are still available as one of the components in the vacuum pump repair kits.

The dual diaphragm vacuum pumps in my 78 and 79 W116 300SDs have 5 check valves in each unit.
Two kits are needed to rebuild a dual diaphragm pump
The 000 586 13 23 kit has one check valve.
The 000 586 12 23 kit has four check valves

I just rebuilt a junkyard spare dual diaphragm pump to install in 'Stormcloud',
and can send one of the good used check valves your way for minimal cost.
Send me a PM.

I just need to keep two in reserve for the 1980 W116 300SD upper oil pans
(that I have already pulled) to be installed in 'Desert Rose' and 'Stormcloud'.
I don't like that the crankcase oil vapor condensate is fed directly into the turbo air intake duct.

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