Yes. Master cylinder replace but that was after the previous problems. Seems like I'm chasing symptoms and not the cause yet. 100 miles to go..... I'll have to take it in again this week....ifbi get these last miles down..

 

Did anybody pull the drums off yet to see if there were any broken parts?

If the master was not bench bled before installation, air trapped in the line to the rears could be getting heated and result in partial application, enough to cause some drag which creates more heat which produces more drag and the cycle just keeps repeating.

I should have picked up on the post you answered where the cables were very loose.
When the springs on the cables , inside the drum, cannot release the brake shoes,they will drag. I still believe those cables are frozen and not fully releasing the rear shoes. Once a drum is removed that will all become apparent. The ideal remedy is to replace the rearmost cables, but sometimes some penetrating oil will allow them to return and work properly .
The best way to prevent this type problem is to use the parking brake and this prevents them from getting corroded and freezing up.