Ventports in the master cylinder help to pump up a low brake pedal by allowing more fluid in the lines. There are several reasons for a brake pedal to be lower than normal. One is that the clearance between the brake lining and the drum is too large. This can happen when the automatic brake adjusters are not working properly. Another reason could be the result of air leaking into the brake system. Vent ports help with this problem because the replenishing ports are not large enough to handle the extra flow of fluid needed when the pedal is pumped.