When backing rear brakes lock up (not always) - I can drive forward a few feet, then reverse again about 5 to 10 feet and the rear brakes lock up - this only happens when I feather the brakes- if I apply no pedal no lockup

Take the time to service and inspect the brakes on every wheel.

Chances are one(or both) of the calipers is missing a bolt, and when you go into reverse, then hit the brake, the caliper is rotating into the wheel, locking the wheel down. When you pull forward, the caliper turns back to where it should be, and works properly. See it all the time. Get the rear in the air, pull the wheels, and you'll likely find one of the caliper bolts are gone....and prolly see some gouging on the inside of the rim. 

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Joel Adams  
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

I have also heard of the rubber brake lines causing that problem. If they are old replace them.

It was a missing bolt on the mount bracket

It turned out to be exactly what you said-had slight scuff on the inside of rim but no problem-after inspection of full brake system found only one bolt under torqued-All is well Thanks

Found a missing bolt on the mount bracket-after full inspection of brake system all is well-Thanks