How can I test my vacuum advance push button 4 wheel drive switch?

I have a chevy S-10 4X4 and the 4 wheel drive worked fine and then the switch quit working in the yard, it appears that the vacuum line isn't activating the button [no resistance in the button preset]. Should I replace the parts or can I test the lines?|||Frist check the trans and grease in the drive unit... The vacum switch no the transfere case ruptures and the fluid is sucked into the vac. sys. fills up the vac ball screws up the a/c, and stuff like that. The act. is under the battery which causes problems too. Check autozone.com and see if they have an online man. for the truck to help you.