Originally Posted by Biodiesel300TD

The vac system is like a tree. The main trunk being the main vac line from the vac pump to the brake booster. Off that line is a few to several other lines(depending on model and year), which then branch more. You'll find that each system has its own set of colored lines. To isolate a system you remove the hose supplying that system and plug the side that goes to the vac pump. Doing this will allow you to see if the other systems are working properly. The systems in the 123 that use vacuum are the door locks, climate control, engine shut off, transmission shift quality, and the EGR. On some models the cruise control is vac operated as well. You Mity vac will work as a vac pump and simple as a gauge. You can pull vac on something, or T it into a system and read what the vac is. Your main vac line should be reading above 20"hg. Nearly everything in the vac system should hold vac if you pull vac on it with the Mity vac. Actuators should actuate when you pull vac and then maintain the vac. If it's not doing both then it's leaking.

I've attached a document that I wrote for another forum about troubleshooting a 123 vac system. I need to get it posted here since the other forum has been closed.