I have a brake booster line from a 1983 240D (apparently same as used in the W123 turbo diesels).

My question is with regard to the second nipple located toward the vacuum pump.
(check valve has 2 nipples)
Is that particular nipple always open or is it closed off? (mine appears closed)
I don't suspect that the check valve can open and close this second nipple.

Does anybody know the function of each nipple?
For example,the first on closest to the brake booster- does it draw vacuum or release..

Likewise the second nipple that I am originally referring to.

Excuse the question if it appears dumb.

Thanks

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD