Small hose to the SAP check value connected to a solenoid switch that should blow vacuum when the car it turned off. if you can blow vacuum into the small SAP check valve vacuum hose, either the vacuum hose is bad and/or you may have a problem with the solenoid (which is very hard to access as it is under the rear of the intake).
But you also have vacuum hoses that feed that solenoid and a check valve that can have issues.
If you want to fix your car, as it is a 1999 model, almost all of your rubber parts under the hood need to be replaced if original. This includes all your coolant hoses as well.
But in the short term, the most common failure points are the lower intake boot, the lower CCV oil return line, the SAP check valve vacuum lines and the DISA O-ring. If your DISA is original, it needs to be replaced at this point anyway.