I had this same problem too, but on another vehicle (a vw bug)
I did the same thing you did, and the only thing left was the
rubber brake hoses, what I found was the inside of the hoses
collapses, only lets fluid into cuz it's under pressure by the force of your pedal but would not let the fluid
return cuz the spring in the wheel cyliner (vw has brake drumsup front) was not strong enough to return the fluid through the collapse hose just like a check valve, and in your case your calipers too probably isn't strong or forceful enough to return
it either, not saying their bad or anything, try removing the hose
and blow in it from both ends, you should be able to blow thru it
if not then you found your problem, and brake hoses are way cheaper than the master cylinder..... just my 2 cents.