PV71 proportioning valve operation

I did the first test drive of my '65 Dart vintage racer project today. Otherwise everything was fine, but the brakes were heavily rear biased. Rear wheels locked long before the fronts.

The car has 4-pot Kelsey Hayes discs at front and 10" drums at rear. The master cylinder is from Mancini Racing: http://chucker54.stores.yahoo.net/ma1bowioutmo.html and there is a combination valve of the type PV71 by Right Stuff Detailing.

It appears the combination valve does not work as it should. Also, it seems that it is not adjustable. I found a picture of its internals, and that got me a bit confused. There is a metering valve, but it is at the front circuit?

Any suggestions of how to proceed (before I'll fit an extra proportioning valve to the rear circuit...)?