clearing vent hose
I suspected my vent hose was blocked. I took my compressed gas air horn, put it on the lower end of the hose, and gave it a couple of blasts of freon. (There is no noise from the horn when used this way.) It cleared the hose right up. For the first time since I owned the boat, I could hear air rushing in through the opening where the vent hose comes out when I pumped out the holding tank.