My brake pedal is getting looser and looser almost to the floor,when driving and I'm hearing a loud hissing noise under the dash,almost like a loose vacuum hose,it almost stops when you apply brake pedal........could this be the master cylinder?.....it just started doing this almost without warning,also back brakes were just replaced but when car sits overnight,large puddle from leak on drivers side rear tire,but no trace of leak from the inside of the tire....very strange,could you please help???,thanks!!