Brake System Difficulties

[P.H. Raker]

Hello, all,     My '63 Greenbrier has a dual master cylinder that the PO says he got from "The Source".  It has always worked perfectly.  Recently that has changed.     If I depress the pedal only once and hold it without allowing it to move at all (exactly constant pressure), it will hold pressure for several minutes.  It will hold either light pressure or very hard (panic stop) pressure.  There are no external leaks in the system. A "hard, harder, hardest" test is easily passed.  This is the way things should be.     If I depress the pedal till it stops and then release a little of the pressure, allowing the pedal to move upward perhaps 1/16 inch, then I can press the pedal significantly farther than on the original push.  If I repeat this process 5 or 6 times, the pedal finally reaches the floor.  Once I release the pedal completely (to the top of its travel), all is normal again.     If I hold exactly constant pressure, the system acts normally.  If I allow a little bit of variation in the pressure but never completely release the pedal (a situation one can easily enconter such as in slowing on a freeway exit ramp), the pedal keeps getting lower each time pressure is increased.     It seems as though when the pedal is released slightly, the pressure initially vents past the M/C seals rather than just reducing pressure in the brake lines.  I don't know how that could happen.  That's just what the symptoms seem to look like.     Anyone have any suggestions? Phil Raker'65 Corsa Cpe - 140/4'63 Greenbrier - 95/4    .