If a vehicle is left in a humid environment, the coefficient of friction between the brake pads and discs increases, generating a discomforting noise during braking called brake squeal. It is assumed that this increase in the coefficient of friction in a humid environment is the effect of moisture penetrating between the brake friction surfaces. Therefore, this paper analyzes the factors causing coefficient of friction variation with moisture between the friction surfaces by dynamic observation of these surfaces. The observation was achieved by changing the disc materials from cast iron to borosilicate glass. One side of the glass brake disc was pushed onto the brake pad and the sliding surface was observed from the opposite side by a charge coupled device (CCD) camera. First, a preliminary test was carried out in a dry state using two pad materials with different wear properties to select the appropriate pad for observing the friction surfaces. Then, the characteristics of coefficient of friction variation when the disc material was changed from cast iron to glass were identified using two pad materials with a wear properties close to that of the selected pad. This confirmed that the coefficient of friction variation was qualitatively the same for the cast iron and glass pads. Finally, the results of the observation of moisture on the friction surfaces found that a mixture of wear debris and water had a large effect on coefficient of friction variation.