Originally Posted by 48bomb
...pointing his finger at .. the wrong master cylinder for this application...
Sounds like it to me as well.
What is the original MC bore diameter and the new one? likely you have an undersuize MC and it takes a full travel to move enough fluid to activate he brakes. The correct bore size will move enough fluid to work the brakes and have plenty of travel left to spare.

Also you said the bracket is flexing, that doesn't sound good, look at fixing/bracing that.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-