A new observation which may be a reason for abandoning the idea of a non return valve:
This weekend we trained scoop rescue. I was the last "victim" in row, so when I was rescued and my cockpit was almost completely water filled, I just strapped on my sprayskirt and started paddling with the group, letting the pump do its job. (A very satisfying feeling, by the way!)
While I was paddling, the vacuum in the cockpit became so high that the sprayskirt was sucked almost to the bottom of the kayak. Suddenly, I heard a quite forceful swooshing sound as air was sucked into the cockpit, and the sprayskirt returned to its normal position.
At first I did not realize where the incoming air had come from - I actually thought that the vacuum had been large enough to burst my sprayskirt somewhere - but then it occured to me that it was of course the pump which had stopped pumping and the air was being sucked backwards through the pump outlet.
If I had had a non return valve in my outlet hose, I would have had to vent my cockpit manually by releasing the sprayskirt.
Eventually, the pressure would of course have been slowly normalized through whatever leaks I have may have in the sealing between the sprayskirt and cockpit coaming / drysuit. But that could have taken a very long time, given that the slow pump was able to create the vacuum in the first place. (Again a very satisfying feeling, by the way: It seems that my Snapdragon sprayskirt works!)
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Haha, good point Allan. I was talking to a couple of old sea dogs on a beach on the Menai straights about my pump install. They both had pumps fitted and strangely enough, warned me about having to 'crack' the sprayskirt to avoid a vacuum in the cockpit. To be honest I found it a little difficult to believe that such a small pump would create a vacuum but when I thought about it, it is pretty obvious that the volume of water being removed from the cockpit is considerable and has to be replaced by air at some point or a vacuum will indeed be formed. Thanks for raising the point, I will hopefully remember to crack my sprayskirt as I paddle away with that satisfied smile on my face.