Vacuum from supply side of a 2 pipe steam radiator while boiler is operating

SamEnergyPlus

Hi all
I am dealing with an issue that a few radiators from a building that we have replaced all steam traps, are not getting hot. It's randomly throughout the building not a specific line,

When we came back we checked the trap if it's failed closed but it wasn't, then we checked the shut off valve and it was completely open, the pipe till the valve was burning hot, so we decided to open the union from the shut off valve, and we basically had a strong vacuum from the supply side while the boiler was on for heat. Does anyone know why this happens and how to fix it?

clammy

Are other radiators on that same main working properly say take offs before and after issue radiators . This 2 pipe system is it using any vacuume pump return ,contain f and t traps and condensate pumps and all the good stuff suppose that comes w it .Or is this a gravity return system , if so are the main vents working same goes for any on the dry returns , do you know if the system pulls a vacuume at the boiler or in the piping ,have you seen any vacuume breakers and is this all radiators w no zone valves or contain steam coils possibly in air handles ? Has the dry returns been checked for any pressurization ,after all that what kinda pressure is it set and running at ?
Peace and good luck clammy