Do you still have an original master and booster? Seems odd that the pedal would have pressure then fade as you pushed harder. Almost sounds like the booster isn't holding pressure or regaining properly. Air in the system is usually mushy right from the top.
Not exactly sure how to test a booster, I'm sure there is a way to get a vacuum gauge on it to see if it is bleeding pressure or struggling to regain.
-Tim
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