Originally Posted by EaOutlaw1969

To test for a bypassing master cylinder all you need to do is as follows. ( from Alldata DIY )

Bypass Condition Test
Disconnect the brake lines at the brake master cylinder.
Plug the outlet ports of the brake master cylinder.
Apply the brakes. If brake pedal height cannot be maintained, the brake master cylinder has an internal leak and must be rebuilt or replaced.


Another function to test for is Compensator Port Check

The purpose of the compensator ports in the brake master cylinder is to supply any additional brake fluid required by the system due to brake pad wear and to allow brake fluid returning from the brake lines to the brake master cylinder to enter the brake master cylinder reservoir.
The returning brake fluid will cause a slight turbulence in the brake master cylinder reservoir. Turbulence seen in the brake master cylinder reservoir upon release of the brake pedal is normal and shows that the compensating ports are not plugged.

Generally speaking the front part ( closest to the front of the truck ) of the master cylinder is for the front brakes.

If your fluid in your master cylinder was neglected before this brake work you may have stirred much of this garbage up during bleeding.

If this was my truck I would replace the master cylinder and three rubber flex hoses. then flush at least a large full bottle of brake fluid through the system.

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