Which tap do I use to top up water pressure?

The water pressure to my Systemate iii tends to drop to zero over time (can usually tell because you get a sound like whalesong in the pipes!).The last time I called a plumber in, I noticed that they turned one of the two black taps near the base which fixed this immediately, but I can't remember which one. Is the smaller or bigger one?

You probably have to turn them both on at the same time....

Cheers, Mike