Some relevent questions about starting air valve-

1.     What is the function of vent?

Smooth operation of the v/v if in case any pressurised air leaks past the piston

2.     Function of balance piston?

Balance the valve and keep the valve in close position when pilot air not supplied

3.     Why balance piston dia small and operating piston dia big?

We know force = pressure * area. As the pilot air pressure is less so area must be high enough to give enough downward forse