In article , Jeff wrote:
The issue is that a considerable vacuum is created while pumping. There
should be a pinhole in an intake line loop to serve as a anti-siphon
vacuum break. This also bleeds off the vacuum in the bowl, and allows
the bowl to fill after pumping, but how fast this happens depends on how
far the bowl is from the loop and the size of the hole. With no
pinhole, it can be impossible for someone unfamiliar with the system to
open it; as I said I can usually pry mine pretty easily but my kid can't
do it within 10 minutes of a pump.

Has no-one thought of a something like a pressure-cooker sprung valve in
the lid - only in reverse? Just press to let air in and break the seal.

Justin.

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Justin C, by the sea.