vacuum coupler

I call it the vacuum coupler because I do not know the actual name of the part.
It is under the dash of my '90 Grand Marquis and it couples incoming
vacuum tubes from the firewall to vacuum tubes going to the doors of the heater. defroster, etc.
First, if anyone knows the actual name of this part that would be great.
Second, I was wondering if this "coupler" ever bleeds internally? All the doors are working fine except the defrost only setting. The a/c, vent, heater doors work fine. In the defrost only setting, not defrost and floor, but defrost only, I get defrost coming out of the floor vent also and I was thinking that this coupler had some kind of internal problem since all the others work fine.
TIA for any replies!
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