A water central heating system is a closed loop system that is under pressure. This means the water in it is circulated around and around the system and is cut off from other water supplies under normal operation. Naturally, slow leaks happen and gas can enter the system in various ways so occasionally this needs to be released from the system. Any gas in the system naturally collects at the highest points along the path - which tend to be the radiators.

When you bleed a radiator you are opening the system to the outside and hopefully where the gas has accumulated. Since the system is under pressure it forces the gas out of the system to equalize the pressure with the outside. This will cause the pressure of the system to drop and eventually it will stop.

However there should be a control valve somewhere, typically on/near the boiler that connects the central heating system up to the mains water supply. You can open this valve to cause water to flow into the central heating system and pressurize it and really this should be done every time you bleed the radiator a significant amount.

In apartments though you might find that you are on a building wide circuit, or you might have one isolated for your apartment. If you have a boiler in your apartment then you are likely on a closed system and should be able to equalize the pressure yourself. If it is building wide you need to talk with your building manager.

Note that you should not need to bleed your radiators that often. Once every several years should be more than enough. If you are doing it frequently then you likely have a large leak in your system and likely want to get someone to check that out.

Having lived in a house with radiators, bleeding registers is like a once every 5 years type of task. There's a much larger leak in the system, you should talk to your apartment manager.

Plumber here.

It's a closed circuit. When you're bleeding out air the pressure in the system drops. When there's no sufficient pressure the water wont reach the upper floors.

Under normal circumstances the system is bled of air once and then they add more water untill the optimal pressure is reached and that's it. You shouldn't need to bleed air or add water ever again. The fact that you keep bleeding it indicates that new air is introduced somehow. Usually this means there's a leak and someone keeps topping it up with fresh water which contains air that then need to be bled out again.

Alternatively the issue could also be non-plumbers like yourself who keep bleeding the radiators on their own and then the maintenance keeps adding more water and wondering why the pressure always drops again. In my part of the world it's forbidden for the residents to bleed their radiators for this very reason.

Also, the radiator lines are usually steel. The reason they don't rust is lack of oxygen. Now with constantly more water and thus air being added to the system the corrosion keeps on advancing.

I'd expect that the person to ask would be the building manager.

That depends entirely on how the system is set up. I would only ever consider increasing the amount of water in the system if I was absolutely certain that my apartment was on a completely separate and closed circuit than the other apartments. You should contact the janitor or whatever it's called.

How do you know how much it's filled?

Also, hot water heat works differently than steam, and many steam systems have been converted to hot water, so it's not easy to know just by looking at a radiator.

As other people have said, let the maintenance folks handle it. They know the system.

I have only done it with steam type and honestly I felt it was one of those more art than science things. buying the adjustable valves was nice to control temp a bit.

There needs to be space someplace in the system since water expands when it warms. Sometimes that space is in the radiators andso bleeding needs to not let all the air out, for this reason you should get maintenance toefix the problem if one exists so they can balance the system.