> The fluid circuit is really a closed loop.  


Yes, true. And because of this the reservoir has more or less fluid in it depending on whether the gear is up or down. If there is no venting the tank will pressurize. There is a very tiny hole in the threads on the filler cap neck. Make sure that this hole is clear. I barely tighten the cap to make sure that I don't defeat the vent. If all else fails, give this a try.

Bill Harrelson
N5ZQ 320 1,350
N6ZQ  IV  awaiting our return from Florida