[Freoway]

Hi folks. Just rebuilding my '57's brake booster. I understand the general principle of how the vacuum provides power assistance, reducing the pressure required at the pedal, but I'm not sure of what the control valve(?) does. There's a bypass tube from the back of the main diaphragm chamber to the front. It attaches to the control valve, which has a filter at the front of it. I opened it up - it also has a diaphragm in it, which I've damaged simply by opening it up. What does this secondary diaphragm do? Can I buy a spare anywhere? Thanks in advance.