kevinski Use a careful low pressure flush,it''ll act like a water heater you'll get some dirty water then run clean---not even close to clean !!IF IT FLOWS AND HEATS GOOD better leave it alone. On your block some time when you have the head off you can hurt that sludge that piles -up around 5 and 6 with a big screwdriver and hammer. Outside of that be glad it will run cool !!
Got the heater pulled apart and found a few surprise’s.Anyone change the hub on the squirel cage the origional had a wood screw screwed into it and the motor shaft was brazed with melted the alloy hub .I think I can drill a bushing and tap a set screw then weld it on the squirel cage.
JW51, I don't know about 54' but my 49', which has the same heater, has a valve control knob on the dash to control flow of coolant through the core. I can shut it off completely or adjust the flow. It has an air filled flat canister about the size of the diameter of soup can with a copper tube back to the valve that will help regulate the flow based on cabin temperature. Kind of an automatic temp control. All factory.
Unfortunately my heater control disappeared so I was going to use a modern valve .The defrost control had two types one a leaver on the unit itself and the other by a cable that I think was under the dash by the heater switch.