I haven't had very good luck with repairing steam radiator patches from the outside (sample size: 2). Steam under pressure is very good at finding microscopic holes between patch and substrate, and enlarging them.
What I have done successfully is an inside patch with epoxy, using a wire brush and steel wool to prep the surface. Getting at the leak may be a challenge, particularly with a radiator that has more than 4 sections you may need to break the sections apart (if you do, this is an excellent time to get the exterior sandblasted and painted).
Simplest and most cost-effective solution is probably to find an architectural salvage shop and replace the radiator (they might take your old radiator as a partial trade).
To get your landlady interested, point out that the radiators are filled with moist, warm air, and that the system is corroding from the inside, shortening the life of the radiators and clogging the boiler with rust. That means the boiler's life is being drastically shortened and may soon cause a massive expense. Fixing it now would be much cheaper than fixing it later. The question is, is she greedier than she is lazy?
As to what's actually the problem?
It seems to me that your apartment's system wasn't being replenished with water. Opening the radiator bleed valve wasn't doing anything because there was no additional water available to replace the air.
Central heating systems come in two flavours -- open vented and sealed -- and each refills with water in a different way. In an open-vented system, there is a tank at the highest point in the system, that uses a ballcock to fill up with water: see this diagram at diydata.com. The ballcock is in the feed and expansion tank. If the ballcock isn't operating correctly, the feed and expansion tank doesn't fill with water, and so no extra water will be available to fill your radiator. The ballcock could be stuck, or it could have been tied off so that some work could be done, and never untied. If you've got that kind of system, the fix would be to get the ballcock moving freely again. Is there some kind of maintenance area you could check out?
Alternatively, it may be a sealed system; there's another diagram showing that. These systems are filled up with a tap -- see the 'filling stop cock' in the other diagram. Normally, you'd check the pressure in the system every few months and use the filling stop cock to make sure the pressure is high enough -- say, 2 to 3 bar. Again, this will probably be very close to the boiler itself and so would need access to your building's maintenance area.
My guess is that it's a sealed system, and that the internal pressure has dropped right down to atmospheric pressure on the fifth floor. Lower apartments won't be suffering because gravity gives them more than their fair share of water, so most tenants won't suffer. However, any rust generated in your radiator will sink down into the system and clog it for everyone.
Best Answer
This is really a bad spot to be leaking considering that it's only about 12 years old. It's leaking where an attachment fitting is welded to the tank and with the amount of corrosion around it, a temporary fix isn't a good idea. It's rusted all the way around and it could start leaking a lot more without any warning since there is pressure in those pipes. You really need to think about getting that replaced.