Just serviced and repaired a leak in my 3 zone system with separate domestic hot water heater. After a drain down and flush to remove air it appears that there is gravity flow in all 3 zones when oil burner fires up. All return lines at circulators are cold but first radiator in all 3 zones are hot. Tried turning all three thumb screws to raise and lower the brass weights and rapped them with a hammer. No luck. What gives?

Are these zone valves you are pounding on? Does the system have a flow control valve. Your gravity heat can be going up the return lines and heating the rooms. And heat backwards. The system can naturally run backwards. When a boiler /heater kicks off the water expands in the boiler/ heater and take the path of least restience. Gravity flow will then rake place. Have you ever had this problem before. If it's a zone valve call for heat and put your ear to the zone valve or metal screwdriver and then ear and listen for its operation. Are you sure you have everything bled out? Air expands more than water and creates pockets. Need a little more information.