Baer Front Disc brake problem

[shevelle6t9]

I have a set of serious street front disc brakes on my 69 chevelle. This kit is sapposed to work with my stock drum parts(lines spindles proportioning valve). But under hard braking the back wheels lock up, the front brakes wont even hold the car to do a brake stand in a water box. I have bleed the lines numerus times and i have even put an adjustable propotioning valve on the rear brake line. i have know idea what the problem my be. I was wondering if any one else has had a problem like this or what you guys think may be the problem.

Thanks Mike