[DeTomaso] removing second set of stock rear brake calipers?

[Larry Finch]

I would not re-install a proportioning valve.

Bear in mind the original system proportioning valve was designed for  
the original tires, wheels, overall weights of them both, etc.

Once you change those, as you have, the balance of the system is off  
and the valve is not dialed in for the new realities.

I agree your drag issue may go away with caliper rebuilding.

Larry