Vapor filled the system under vacuum starting from nearest radiator - A.

When this radiator warmed up (pressure is up) – condensate close steam trap and to next radiator, etc. – B

The steam trap behind each radiator releases condensate back into the boiler, but prevents steam from entering the return line and vacuum pump. Steam traps present a major vacuum system maintenance problem, because they last only 10 years at best. Leakage through a single failed trap ( out of hundreds or thousands - for example, 6600 in Empire State building) destroys nearby steam traps like an avalanche, overloading the vacuum pump, condensate pump, etc.

If any steam trap fails open (out of 100s/1000s) - C, hot vapor get into return lines, overload vacuum pump and hit/destroy nearby steam traps. The result is an unbalanced, noisy, and very expensive system.

In the known vacuum technology only condensate return lines behind steam traps are under vacuum. Steam traps failure is a reason of high maintenance cost and bad reputation.