Not sure which car you're working on (you might consider adding it to your signature line so nobody has to phone Ms. Cleo to know....)

On the 126 the check valve is in-line with the washer line to the nozzles. Follow the line from the pump to where it splits, usually right behind the headlight and you'll find the check valve in the form of a "splitter".

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]