bought a 67 cpe, 289, C4 a couple months ago.  A previous owner installed the SSBC power disc conversion kit (A132-A). 
https://ssbc-usa.com/products/a132-a?rq=yr_1967~mk_ford~md_mustang~ha_base
The guy i bought it from hasn't used it much in the past 4 yrs..  Now I'm battling some front brake drag.  The fluid was badly contaminated so i flushed it thoroughly with a pressure bleeder.  It is now clear & bright at all 4 corners (factory drums still on the rear).

I've also removed both calipers, disassembled the piston (single) and used 600 & 1000 grit wet sand paper to clean up the cylinder and bore.  I've done similar work to the slide pins.  This past weekend, i disassembled the master cylinder and cleaned out the bore like I did with the calipers.  

Even though I had a great 20 mile test drive on Sunday, the problem came back again today.  I had to slow down to about 35mph to ease my way home this evening.  

I checked the temperatures after I got home and the discs were both hot (300+ deg), but at least they were only about 10 degrees different so i don't think the calipers are hanging up otherwise one would probably have been much hotter than the other.

So... Before I just start replacing parts (calipers & master cylinder), is there anything else to look at?  Could the power booster hang up somehow?  The distribution block appears to be original.  Rear drums are working fine.

As always, any input is welcome.