Thanks Diesel911, your insight makes sense.

Yet there seems to be a mechanical difference between the two nipples.

So directionality does exist since the two nipple check valve does have an arrow designated "to motor" for proper installation.

The difference between 0.5 and 0.6 does not seem extreme, that is why I am befuddled about the 0.5 black nipple not allowing pressure by mouth as the 0.6 white nipple does.

I can't figure out because at this point I am not educated on the actual vacuum behavior designed for that 0.5 black nipple.

Anybody?

__________________
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