[65comet]

There is a leak somewhere. Start under the dash to see if the fluid is coming back in from the master cylinder push rod. Sometimes it is hard to see that leak when it is going down under the carpet. If not there, then follow each brake line, paying careful attention to the points where it may be rubbing against something else. If you have rear drum brakes, the leak could be inside the drums. The fluid won't evaporate, you just have to find the wet spot. Have someone else press on the brake pedal. A leak will usually make a hissing/spraying noise to help locate it.