Disc Brake Conversion Failing

Also the OP Stated a swap to 68 Disc's. This would imply Kelsey Hayes.

As a kinda less the happy KH owner since 1984 there are many issues that can be observed. 1" master is the best.

A "REAL" factory Master would be heaven! They have been remanufactured and given every "lie" in the book as to actual purpose.

If, one can purchase a 67-71 "True" A-body Disc/Drum master, more power to you!

Will they work? Well maybe, Will they fit correctly "Specifically BB Power Disc"? Doubtfully! Can you find one that will get it done? Yes!

My best response is go to the store and order every one of them listed for the application. Make them take each one out of the box and check for overall size, reservoir dimension and more before you buy it! Most of what is sold today is pure junk!
Flip over the MC and a 1" bore should have a 411 at the end of the part #. This has no barring to the rear brake lock up. Contamination of fluid in the line or on the shoes or a leak on that one side. When bleeding, you need a steady flow of fluid or you may have a blockage of some kind. Hint: When replacing any WC I always pump fluid through the lines before attaching new part.