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It sounds to me like the O.P. may actually haver a check-valve in place, not two anti-siphon valves.
If this is your one and only, or primary bilge pump, I would get rid of the check valve.
As for the reason why water is siphoning back into the boat, I would suspect that your anti-siphon valve is gunked up with salt or other deposits and is not functioning properly. I.e., it is no longer open and therefor isn't breaking the siphon. You should at least confirm that it's working properly.
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT