Classic/Antique Car Repair: 38 Olds Brake Problem, vent holes, wheel cylinders

Question
I just put a NOS master cylinder on my 38 Olds and my brakes worked for about 10 miles.  I slowly lost my pedal and I cannot get any pressure back.  I noticed that fluid will come out of the vent holes on the master cylinder cap.  Time to get my NOS cylinder a rebuild job or a new rebuilt cylinder? Thanks!

Answer
Believe you have line pluggage or corroded cylinders. The fact that it worked breifly was all of the cylinders getting pressed out. You problem is return of the fluid and that is blockage. Have you purged the lines all the way to thru the wheel cylinders individually?