Is there a valve under the faucet that was partially turned off? Is it cold only, perhaps a drinking water filter? Could the sink faucet valve be plugged?
I suppose reverse pressure may work but you need to take a line loose somewhere within the system to release it. The external water hookup and the pump have anti-reverse valves. And I wouldn't use much pressure, check to see max pressure for the system (65 psi comes to mind).
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles
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