So I have a 66 C-10. After sitting for a couple days, I have no brakes and no resistance in the pedal. If I take the cap off the master cylinder and put it back on, I have a rock hard pedal that instantly locks the rear wheels up when pushed. All original parts. No leaks that I have found. Any help would be great. I would like to fix it myself but not sure what the problem is. Booster? Master cylinder?