I believe, but am not certain, that what you're looking at is a check valve to prevent
water from above the valve from flowing back down into the bilge.
If you have a check valve well above the
pump, and the discharge well above the check valve, after the pump stops pumping, the
water below the check valve drains back through the pump into the bilge. At this point you have, starting from the bottom: the pump, then a length of hose with air in it, then the check valve, then a length of hose with water in it, then the discharge.
The pump has a hard time priming. The slug of air is trapped by the slug of water above the check valve. The pump is not designed to pump air very efficiently, so it can't push the air hard enough to
lift the slug of water above the check valve out of the way.
One solution is to have a tiny vent hole *below* the check valve. This allows the trapped air to escape when the pump is trying to prime. The downside is that when the pump is running, of course, a tiny percentage of the discharge flows out through this vent hole rather than through the main discharge. It's usually piped so that this "wasted" discharge runs harmlessly back to the source.
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