The topping up of the cooling system is becoming more and more difficult. Whereas with oldtimers, there was a relatively simple cooling system with a coolant- and a heating circulation, indeed, the number of circulations in modern vehicles is ever increasing. Thus, as it was in the past, it was generally sufficient to top-up with coolant while the engine was running, nowadays, this will no longer guarantee a completely air-free cooling system.
In addition, something new has appeared in the field of eqiupment, this is the vacuum pump which is operated by over-pressure. In this case, the Venturi-effect comes into play (see picture) which has been that much refined, that with a compressed-air system producing 6 to 10 bar and a delivery rate of 125 liters/min., an absolute pressure of 0,8 bar can be generated. We can also, by the way, thank the new technology for the relatively strong suction-jets which can be used for almost anything in the workshop.
You can almost imagine what happens then. A compensation tank with the respective airtight cone is connected to the system where the coolant has been drained off, it then sucks up until the 0,8 bar are reached. The tube to the coolant container is then opened, thus filling the system without leaving air in any of the cavities.
By the way, you can of course, maintain this pressure for quite a while. This is also quite a good way to pressure-test the system. Although additional leaks can appear under high pressure, it is basically a practice orientated testing method. 09/13