Two Radiators staying cold

booyah2k1

I recently moved into a home that has floorboard radiators and a boiler for heat. Winter hit and everything seemed to be working fine for the first month or so. But a couple of days ago two of my radiators stopped getting warm at all and stayed cold. I live in a ranch and have 9 total radiators on the main floor. Seven of them continue to work and get hot. I traced the pipes coming from the boiler and there are 4 that go upstairs with hot water. Three of the four get very hot to touch while one of them remains cold/luke warm. The valves are open on all four. I have done some research online and read that one of the issues may be air in the system. So I thought I would find a bleed valve on the radiators in question. After taking off the covers I realized that they never installed bleeding valves on any of the radiators. So I went back online and read that you can bleed the system from the basement boiler with the hose hookups. I was hoping it would be straight forward, however I have found that my system has two drains and am unsure on which one to use. Do you guys have any advice?
Thanks!