I have a 03 silverado yesterday the brake pedal went to the floor and truck started moving while at a drive thru,i was able to stop but had to pump the brakes. I have not lost any brake fluid and have bled brakes. The brake pedal goes down almost to the floor only about 1 inch of brake pedal and doesn't stop very well. I was thinking master cylinder but I had plenty of fluid coming out when I bled them. How do you check brake booster? Help thanks joe