When you pump it up, i would pump it up until the guage shows about 20 -25 bar. I think i read somewhere that the actuators engage and disengage at about 5-10. I always pumped mine up around 20, it doesnt take much.

You could swap out the solenoid, its only like $30-40 and would be an easy fix if that is the problem. Even though the 4ws seems to be going in and out fine, the solenoid can still be the cause of leak, the seals in it can be shot and allowing the vacuum to leak down.