Water comes out the back bumper! 1972 Argosy
Welcome to the forums!
Another possible culprit is the water pressure regulator valve. It may have frozen (corroded) or been blocked by sediment or hard water deposits causing it to stop water flow and redirect through the relief port which can cause the condition you describe. Finding this valve can take some gymnastics as it is usually well hidden in the streetside corner closet/wwardrobe on the floor.
Good luck with your investigation!
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)