I rebuilt the Treadlevac Master cylinder in my '58 Special, now it builds pressure somehow as you drive, until the point it'll start to apply the brakes...the pedal gets higher and firmer, until it becomes almost solid and can't even be pushed...I found when that happens I take off the Master Cylinder fill cap and push down on that valve in the center inside the cylinder, and it relieves the pressure (if it's quite pressured up, the car will actually lurch or twitch a little when the brakes ease off!) and then I'm good for awhile...but it'll start building again as you drive. It seems to happen about every 40 miles or so.
Any thoughts on what do do about this?? Besides convert the car to a normal master cyl and booster, which I may have to do?