i had the same problem with my 83' 300 but i think i figured out what my problem is. not sure if its the same for you but i've been using the wrong type of fluid for my breaks. i added some dot 3 about 2 weeks ago and the problem like yours started up and the proper fluid should be dot 4 i didnt realize this until recently. but adding the right fluid fixed the problem. hopefully your prob is as smiple as mine