I have a Burnham series 2 boiler FHW system. Today I tried to purge one of the zones. The boiler was at temperature, I turned on the pump for the zone, shut off the return valve for the zone, and opened the drain valve. After a few gallons had flushed out, there was a lot of shaking and the flow stopped. The fill gauge on the boiler was at zero and there was no heat going anywhere. After about 30 minutes of thinking I had done something terribly wrong, the fill pressure started to rise again, and everything went back to normal. Is this normal, did I do something wrong, or is there a problem with the boiler's autofill?