A PVS (Ported Vacuum Switch) has many different uses depending on application. If this were a Ford engine there would be a calibration code for the engine and all of the emission parts could be looked up including the vaccuum hose routing. I assume GM has a similar system. Then you will have to find some one that would have a book for a 24 year old GM engine listings.
OR you could follow the vacuum lines to the devise or devises this switch controls. 2 of the lines will likely pick up vacuum signals from different sources. One line goes to a devise that is to be controlled or operated. The 4th line ..... not sure. Once you find what it controls the switch can be searched for using that description. Meaning the devise controls a xxxxxx. So you would be looking for a PVS that controls a xxxxx for a 1987 GMC with a xxx engine size.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!