Rear brake bias problems - Viper front calipers with rear drums

Hi all, Merry Christmas.

I'm trying to iron out some brake issues on my '67.
-Rear tires lock up before the fronts
-Brake warning lamp comes on when I hit the brakes

Here's what I have (nothing is original):
Manual brakes
15/16" master cylinder (new from Autozone, intended for a front disk/rear drum mopar)
Wilwood adjustable prop valve with warning switch
Front: 11.75 rotors, 1st generation Viper 4-piston calipers with AR Engineering brackets
Rear: Stock 11" drums from a 70's van, 10lb in-line residual valve

As for the rears locking up: My proportioning valve is adjusted all the way out to cut as much pressure to the rears as possible, but the rears still lock up first. I think I may need to go to smaller wheel cylinders in the rear. Any thoughts?

And for the warning lamp: I have bled the system repeatedly. The pedal is pretty firm and doesn't lose pressure. It doesn't feel like I have any internal leak in the master cylinder.
My plan is to bench-bleed the master again. If that doesn't work, I'm gonna suspect either a bad master cylinder or just a bad mismatch of parts on my behalf.

I'm interested in hearing from the experts!
Thanks,
Scott