Dodge Caravan Brake Pedal Spongy

After About 8-10 Minutes On The Road At 55 Mph I Come To My First Stop Light. When Braking I Smell A Burning Smell Through The A/c. Then...

Brakes Failed At A Three Way Stop. They Went To The Floor With No Stop. I Bled The System Roadside And It Were Good For Three Stops And Then...

I Replaced The Rear Cylinders And Bled All The Lines (several Times) But The Brakes Are Still Soft And At Times While Pressure Applied Just...