95 s10 blazer vacuum problem

Have a 4 door 95 s10 blazer that won't go into 4 wheel drive. Replaced the actuator, but it acts like it doesn't have enough vacuum to keep the front differential engaged. Is there any areas to check for vacuum leaks that would cause this? Replaced the actuator twice, already, and I don't really want to make it a manual shift. If anyone has any ideas, great.

If vacuum is weak at the actuator, trace the vacuum line back. It goes to the transfer case switch. From there up to a reservoir under the hood, then to a vacuum check valve, and on to the intake manifold. Not a big deal to replace all of the lines. Make sure the check valve is good.