Sounds like its the siphone for the other side of the tank. It shouldn't be loose but that's a planned/ expected leak if so and shouldn't stop priming. Wouldn't expect it to hold a prime though.
The stock pumps have it built into the pump outlet. Dual barbs. Glad that's all it was. I'm surprised that setup bleeds that much off that it won't prime enough to start. Stock pump orfice is small.