Originally Posted by 2.5Turbo

That's the vacuum amplifier.

From http://www.peachparts.com/Wikka/TransVacTune :

"The most important modification came with the 1985 model and is continued on all subsequent ones. It involved the addition of a vacuum amplifier to the system (see Fig. 7). The amplifier takes a large vacuum supply, the proportioning vacuum signal and a boost signal and creates the final signal to the transmission. This device did two things. First, it incorporated the input of boost to further tailor the load control of the modulator vacuum signal. The vacuum at the modulator now starts high - 12-17 in. - and is reduced to about five inches through straight throttle movement. As boost builds, the vacuum is further reduced to zero only at full boost."