745 TD servo brake vacuum hose t-pipe cap?

After waking up my 1989 745TD after winter break I noticed that the servo
support of the brake did not work at all. I had noticed during the last winters
that during cold weather that the vacuum took some time to build up compared to
summer temperatures, I was afraid it was the vacuum pump. I checked the vacuum
hoses now and noticed a t-pipe between vacuum pump and servo brake unit. On the
third outlet pipe was a rubber cap with a hole in it. Vacuum could be felt when
putting a finger on it. So the vacuum could not build up with the hole passing
air in. On my VADIS DVD I found the t-pipe but not the cap. I went to a Volvo
repair shop, same there despite newer program. The spare part seller was really
nice, went up one floor and retrieved the repair manuals on paper for that car,
no cap on the images there either but it became clear that the whole T-pipe is
there for attaching a manometer to measure pressure leaks in the system. So
under normal operation this tube should be closed. I suggested that we might ask
an older mechanic in the repair hall - "there is no older guy out there" and
"our Volvo mechanic barely sees such a car once or twice a year" so there was no
possibility to check that way. The spare part person offered to close the hole
in the cap with a little plastic nipple filled with instant glue. I put it back,
servo brake works again. So my question: Does anybody know which kind of cap is
there, what is in the hole of the cap originally (apparently something had
fallen out of the hole in the rubber cap)? Maybe it´s best to replace the t-pipe
by a straight pipe?

Viktor
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personal e-mail vikwei at gmx dot at