2004 P71 brake issues
 

for about the last week ive been having what im assuming are the back breaks on my car locking up randomly tracked down by a burning smell to the back of the car. Ive replace the rear calipers rotors and pads thinking the calipers were at fault but did not change hoses and my fluid level is good. both sides seem to be applying themselves even when i havent touched the breaks for 10+ min also the break pedal gets rock hard with very little travel to activate the breaks. any help would be greatly appreciated as this is driving me crazy thanks