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Can you explain in more detail what is going on ? Are you getting good fluid flow at the four wheels? Is the pedal spongy? Why was the master cylinder replaced?
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With the lines off at the master cylinder and holding your fingers over the holes do you get good flow and pressure at the master cylinder if you have someone pump the pedal?
Sorry they block numbers not permitted off site communications. Make sure you can get some pressure at the master cylinder it is not unusual to see a bad after market master cylinder. If you can not get pressure at the master cylinder holding your fingers over the holes after someone pumps the pedal 5 or 6 times the master cylinder is no good. Gravity bleeding is not always a good way. There is a LSPV on the rear of the truck that has a separate bleeder on it that has to be bleed. It will get air trapped in it or can stick and keep fluid from getting to the rear wheels.