Most likely it is the POD, I will say it is a couple of hours DIY job. I did that last weekend, it took me all day but I also took all the AC ducts and blower motor apart to clean it.
The best way to start would be take the ACC console, radio, ashtray and center cosole out. There is a 7 port vaccum switch thing right behing the centeral cosole /ACC button wood trim. It has all the vaccum lines from pods connected to it. You can test all the lines for any vacuum leaks with mighty vacc or by sucking with you mount on it and placing a you tounge to hold the vacuum. Which ever method you prefer, mighty vacc is nice because it also gives you the reading of vacuum on the dial. Based upon which line is not holding the vacumm, you can follow it to the bad pot, you lines could be leaking too. My suggestion would be to change all the rubber L shape connector on the vacuum lines, they usually go bad with age. It will not be a bad idea to replace the PODs or the diaphragm on all the POD since you are there. There are 2 pods (dual chamber) right behind the glove compartment and real easy to access by taking the glove compartment out. The hardest one, even if possible without taking the Dash out, is the center vent pod. Most people change the diaphragm on the center vent in place, it is hard but doable. There is one pod on the drive side way back in the dash and another one on the RHS of the dash.
Have nice cool day, and keep us posted
87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)