I just finished repairing a one pipe steam system installed in a 5600 sq.ft. cottage with 12' ceilings. The system was installed around 1910. The two largest supply runs had the usual air vents at the end of the run. Two of the secondary supply runs had no main air vents and relied on the radiator vents to release the air. However,the last radiator on each of these runs had a return pipe from the radiator down to the return system. There were no traps and it was original piping. I did not see any reason for this, nor have I ever seen it before. Any comments on this?