The problem with having front disc , and rear drums, is the
volume of fluid difference between the two types. Disc brake calipers take more volume than a drum brake wheel cylinder. That's why a master cylinder for a typical disc/drum system will have two different size chambers, with the small chamber for the rear(drum), and the larger chamber for the front(disc)
What is
most likely happening is the first pump of the pedal is just filling the front calipers, getting ready to apply the brakes. It's taking almost a full press of the pedal to do that, and at the same time, trying to apply the wheel cylinders for the rear.
Does the master cyl you have have two same size chambers, or one large/one small? Usually, the smaller chamber will work the rear(drum) brakes, and the larger one working the front(disc) brakes. The P-valve keeps most of the pressure going to the front. Changing to front discs mean you'll also need to change the p-valve, since the calibrations are different between the two set-ups.
IMHO, you(your neighbor) need a dual line, disc/drum master cylinder, and the correct disc/drum proportioning valve. It's not likely you'll ever get a good pedal with the set-up as it is...
In a previous life, I was a Mustang collector...I had 7 early Musturds when I moved to D/FW in '86, including a K engined '64 1/2. I've grown out of that phase, tho...
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