Mine developed the same problem recently. Usually it's the valve itself that goes. First verify by applying vacuum to the unit via the vacuum line behind the brake booster. If it holds vacuum then the valve is good and the problem may be the solenoid relay. The valve is $200, the solenoid is $35.

I just put a small ball bearing in the vacuum line anad live with it. The problem is that I have to clear the check engine light every now and then.