I have a 1980 / 1981 T-33, I never noticed a screen in the tank vent line area. I had to replace the vent itself on the hull because a mud wasp had built their nest in it and blocked the vent line, I broke it removing it . This was about 10 years ago and I think I remember the vent unit having a screen. If you can see a screen in the tank then you may be able to clean it with a tooth brush, etc. You do need the vent to displace the fluids while being pumped out or the tank might collapse in on itself and destroy the tank because of the vacuum caused by the pump out. I suggest you replace the hose between the tank and the vent at the hull . This might fix your problem.
Leo
Leo Corsetti
T-33 Satisfaction