Is the hose to the rear axle new or in very good shape? Hoses can deteriorate inside and act like check valves.

But if you loosened the line at the master cylinder, that says that the master cylinder is the issue. It may not be retracting far enough to release the brakes. Try adjusting the pedal linkage so that it's shorter.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.