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gas furnace won't drain water properly

I have a Goodman GMN080-4 90% gas hot air furnace, about 15 years old. When it comes on, it will run for a while, then shut off before reaching the thermostat temperature. I checked the diagnostic light, and it’s blinking three times, indicating a pressure switch problem. I also noticed water in the tube going from the draft inducer fan to the pressure switch. I removed the tube and blew the water out of it. I also removed the draft inducer and saw about an inch of water in the black plastic collector tub behind the draft inducer. I siphoned this water out, started up the furnace and it worked fine for a while, then the same problem. I think the problem is that the condensation is not draining properly, and it’s backing up into the draft inducer. I have a PVC exhaust pipe with an unusual fitting on the bottom of it. The fitting has a tee that connects into the draft inducer, and the fitting also extends about 8 inches below the tee, where it’s capped. This fitting has two small outlets, just below where the T connects to the draft inducer, and each outlet has a tube connecting to a condenser pump on the floor. It seems to me that I should see water draining down these tubes when the furnace is running; however, I don’t see any water in the tubes. I thought the drain lines might be clogged, so I disconnected the draft inducer and poured water into the T. The water came out of the outlet tubes, proving that the drains are not clogged. I hooked everything back up, and I get the same problem – condensation doesn’t drain, water backs up into the draft inducer, pressure switch opens, and furnace shuts off. Any ideas? Does the fitting at the bottom of the PVC exhaust pipe (by the way, is this a “condensate trap”? It doesn’t look like a “trap”) need to be replaced?

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