Reviving this thread, as I have a very similar - maybe the exact same - problem, but I don't see a totally clear answer in any of the replies.
Here's my situation: shower has a small (1/2 gallon?) sump in the floor, with a Rule 360 gph bilge pump. (Model 24 - it's activated with an external switch, no automatic switching.) The hose goes from the bilge/sump, down about 18" (to go under a bulkead), then back up about 2.5 to 3 feet, to what is, I believe, and anti-siphon lock*. From there, another hose drops down about 18" or so to a thru-hull that's just above the waterline.
*I think it's an anti-siphon lock. It's an upsidedown "U" shape, about the size you would make with your thumb and forefinger. At the top of it is a screw-off "cap", and inside the cap is a small rubber piece that sticks down into the tubing of the "U" shape. I have a picture of it, if that would help.
Similar to one of the other posters: the pump pumps like mad when just in a bowl of water, with the pickup always submerged. The outflow hose is clear, as I can fill it with water and easily blow all the water out. But when the hose and pump are connected, none of the water flows out. The one exception: I had the hose and pump disconnected; I filled the hose with water; I put the pump in a bowl of water and got it moving that water around, but not sucking any air; then I very quickly inserted the out tube on the pump into the hose, and it emptied the bowl in a flash.
Oh - I also disconnected the hose from the anti-siphon lock - the hose that runs from the shower - and put the hose end into a bucket, then tried to pump water out of the shower. Still no go. At that point, the hose ran from the sump / bilge pump down about 18" then back up about 2' to the bucket.
So, I think the problem is that the pump works fine as long as there is no air getting into it, but as soon as you use it, it removes all the water from the sump and sucks in air, and then it stops working again. i.e., I don't think the siphon lock has anything to do with it. I think the problem is that the pump simply won't pump the water if it ever gets any air in it. And yes, I had a brand new spare pump that does exactly the same thing as the old one.
Do I need a different kind of pump? One that's specifically designed to be self priming, or something like that? The funny thing is, when I first started troubleshooting this problem, I discovered a giant wad of hair on the bottom of the pump, which had obviously gotten there over many, many showers during which the pump was almost certainly pumping the water out. So I'm pretty sure that, at some point, this system worked.
Even though my spare was new in the package, it's probably many years old. Should I just buy a new pump and try it?
Thanks.