Sending unit? Do you mean the pump itself? ... just replace the pump. Should be around 50$ or less.
There’s 4 wires on the pump housing.
2 low fuel sensor
3 low fuel flash like hell panic senor
4 fuel pump
....Priming on key on is controlled by the ecu. You key on, it supply’s power to the pump for a few seconds , then kicks off.
after that it’s controlled by voltage from the TPS and ECU....
So if your not hearing the pump prime when you key on...(priming)...chances are your pump needs replaced OR you’ve buggered up your wiring elsewhere.
Your water bottle “test tank” is basically gravity feeding. Your pump is prolly just broken. Sometimes they’ll fail closed or open. Either leaving you with little to no fuel coming out, or gravity feeding.
you prolly just need to buy this:
Find many great new & used options and get the best deals for 97 98 99 00 GSXR 600 SRAD FUEL PUMP & PETCOCK - *NEW* at the best online prices at eBay! Free shipping for many products!
that eBay vendor is also a forum member here.