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The function of back pressure valve is to prevent back flow to the pump in case there is an obstruction on downstream side of valve.
They serve two different purposes. An air bleed valve is used to remove air from the circulation system. A pressure relief valve is designed to automatically open when certain pressure levels are reached.
Raise the pressure in the boiler until the valve relieves and note the pressure at which it does so.
Sounds like you have a bad zone valve or zone pump. There is either a pump, and the zones in the house have zone valves, or each zone has it own pump. Zone valves go bad all the time, you can just replace the motor on it instead of sweating the new valve in. The box that is on top of the valve.
Stop Boiler Valve (or Crown Valve) is installed on an outlet line of a boiler. It's purpose is isolation of a boiler from a downstream steam line. Its position may be either open or close, it cannot be used for throttling of steam.