Classic/Antique Car Repair: 1953 Chevrolet 1/2 ton dump truck, wheel cylinder, air bubbles

Question
QUESTION: The brakes keep failing in the truck.  We have replaced the master cylinder.  Any suggestions as to why it keep happening.  Brake fluid helps for short time.

ANSWER: Please explain "failing". Does the pedal go to the floor? If you step on the brakes do you have pedal but the truck will not stop? Are you loosing brake fluid and have to keep adding fluid? Does the truck have a power booster? Let me know a little more about what is happening and we will go from there.
Brad

---------- FOLLOW-UP ----------

QUESTION: The pedal does go all the way to the floor.  The truck does not have a power booster.  As to fluid, not sure if losing, parked on grass, now snow, but moved recently.

Answer
Ok, we need to do a few things to determine what is happening. First, you say that you replaced the master cylinder. Did you bleed the system after replacing the master cylinder? To bleed the system you need to fill the cylinder. Then put the cap on loosely. Under the truck locate the bleeder screws at each wheel. The bleeder is located on the back side of each wheel cylinder near the point that the hydraulic brake line attaches to the wheel cylinder. The process is to have someone pump the brake pedal a half a dozen strokes and then hold the pedal down. Go to the right rear wheel and loosen the bleeder screw and a squirt of brake fluid and air should come out. Close the bleeder screw and have the brake pedal pumped up again and held, open the screw again. Do this a half a dozen times and then check and add brake fluid. Then go to the right rear wheel and repeat the process. Then the right front and left front. At this time you should have a brake pedal. Now repeat the process watching the stream that comes out of the bleeder screw. Bleed until the stream runs clear and has no air bubbles. Do not let the master cylinder run out of fluid, because if it does you will have to start all over again. Now with a brake pedal, hold pressure on the pedal for several minutes. The pedal should not sink out from under your foot. If it does there is either a fluid leak in the system or the master cylinder has an internal problem. If there is a fluid leak it could be in one of the wheel cylinders, which will require removing the wheels and brake drums to find, or a leak in a line, and that can be found with a visual inspection. Good luck and keep me posted.