Hot water radiator - forever draining

I need to remove a hot water radiator temporarily (so I can install new flooring beneath it). Of course I need to drain it first. It is in the basement, and on the same circuit as six other radiators on the first floor.

At the boiler I closed the valve on the pipe that feeds that circuit. Then I shut the input valve on the rad. (There is no valve on the output side.) The input pipe is too close to the floor to get a bucket underneath it, so I loosened the nut enough to allow a trickle into a shallow pan, then a siphon through a 1/4" vinyl tube leading to the basement drain.

The water has been trickling/flowing for 24 hours. I have no idea how long it will take. I assume that I need to drain *all* the rads on this circuit (since the water in the upstairs rads will flow down into this one). When I bleed the rads on the first floor I get water almost immediately, which tells me they are still pretty full. BTW there is no way that I can tell to drain the water at the boiler.

Any thoughts??