It is entirely possible that the water pressure from the hose has forced the water pump into "by-pass" mode. The pump will not come out of by-pass mode until the residual pressure from the CW line is relieved.
'07 K3500 Silverado LT Crew Duramax (LBZ)
2016 Salem 27RKSS
1984 CHEV SCOTTSDALE K20 2GCGK24J0E1XXXXXX (Chevrolet Legends-Class of 2019)
"...exhaust fluid? We don't need no stinkin' exhaust fluid"