If it has power brakes the fluid from the rear reservoir can drain into the power booster, eventually causing it to fail. Its possible for the fluid to get into the valve inside the power booster, causing it to get sticky. Overly sensitive brake pedal or a hard pedal can be simptoms. If no power brakes, pull the carpet back from the inside of the firewall and look at the master cylinder rod for fluid running down and under the carpet. There may be a rubber boot covering the back of it where the rod goes into the cylinder, pull that back and look for fluid. In either case, you may need a new master cylinder.