Maciek.
I agree with you check drain, was not the best word for the pipes. Venting and air supply is probably more correct. The purpose of the system has two tasks. As you correctly are stating, venting. This is particularly important for the Q tank ( Untertriebzelle port and stb.) which is used for emergency quick diving and silencing is essential.
As the regulating tanks, are not connected to the pressure water compensating system , the pipes are used, when the regulating tanks are used as fueltanks, as air pressurepipes to the tanks in order to transfer the fuel to the internal fuelsystem, as you see it is possible to use this LP air to the saddle ballast tanks 2 and 4 port/stb as well.
Tore