first replace the plastic valve and vacume line, see if that fixes the problem. If it doesn't pull the master cylinder out and bench test it, IE drain it completely fill it up and prime it. Step one, remove master. 2, drain. 3, re-fill. 4, secure the master in a vice or clamp. 5, take the end of a hammer and depress the little metal cylinder on the back of the master. 6, repeat 5 untill no bubbles appear and you have less than 1/8 inches of depression. If it bottoms out and never gets hard to do this then you have a to replace the master ($50-60). If it works then you have a booster problem and replace that.