1966 Valiant Dual Master Upgrade Problem

That sounds all messed up
Will need a pic to sort it out.
Original drums all around?
Original Wheel cylinders?

OK wait I get whatchur saying
And I don't know the solution.
Except to say that you probably have a mismatch of tubes and fittings. The original parts are often coded with specific factory fittings so that it is impossible to plumb them wrong. You may think that the part you are using is correct because it screws into the block, but it is likely that the line is not the correct size to fit the seat. Take a real good look at the original lines and fittings.
You may have to buy a flaring tool and fabricate your own ends.
Is this your block?

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If yes then look at every port and determine if they all take the same tube size, irrespective of the fitting sizes.