switch for front diff lock on 1998 Toyota T100

I have a 98 t-100 4 wd works at times and does not at others. If the 4wd indicator light was on 4wd worked. If i shifted in and the indicator did not light 4wd did not work. I pulled the switch or piston that moves the fork back and forth shifted in and out and the fork did not move. I noticed it is opperated by vaccume. not sure how this works if it is a burst of vac or constant to trigger the piston. once removed I could not detect vac with my finger. Is it more commonly a vac issue or is the mechanisim know to fail. I did not notice any breaks in the vac lines and all were hooked up. The truck also otherwise runs great. I only have 130,000 miles. Thanks in advace for any help

Asked by for the 1998 Toyota T100
Use a vacuum testing pump to operate the vacuum motor that shifts the axle, to see if the actuator is working. You may have to run a large vacuum line that is teed into your brake booster line for more volume. Then if the actuator works, use the vacuum pump to test whether or not the system is getting a vacuum signal to the actuator when you select 4wd.

here is a site that has repair info for your T-100
Thanks so much. I will give it a try this weekend