I have noticed the last couple of weeks that my brake pedal is very stiff/difficult to depress only when starting the car first thing in the morning or after it has sat for several hours (cold start). When putting my foot on brake pedal before pushing the start button, the pedal will barely depress. After the vehicle has started, the brake pedal depresses normally. If I drive my car and park it, turn it off and then restart it, the brake performs normal (the brake pedal depresses when foot applies pressure, and car starts after pushing the start button). Is there a problem in my brake booster/vacuum system? A leak? This issue only occurs on cold starts when the brake has not been used for several hours.