Help me stop!

A distribution block is just that. A Y-block. I suspect you mean, combination valve. The kind with 5 lines attached to it(2 in and 3 out).The combination valve is often called a brake proportioning valve. While it may be possible for that valve to make trouble,its hardly likely. The circuit that pertains to the front brakes is just like a Y-block;that is one line in and 2 lines out.Also bear in mind that this valve is a safety valve , in that if one end fails it turns on the dash warning lite.
-Bear in mind how brakes work. They convert the momentum of the vehicle into heat through friction at the pad/disc interface.Things go wrong when A) the pad cant do it, because it is too small, or wrong material,or B) the disc cant get rid of the heat to the surrounding air, or C) There is not enough clamping force coming from your leg,..EDIT.. or D) one or more of the parts are not functioning as designed.
-Its also possible for some of the brake force to be lost before it reaches the caliper. Lost to flexing of hoses or flexing of the caliper, or even the firewall.The flexing of things is easy to see or feel.The pedal ratio can be figured mathematically. The brake force can be computed once all the bore sizes are known.
-If your pedal is hard, Then the problem is not likely to be hydraulic.
-IIRC factory discs on A-bodys are all at or near to 11 inchers. And the caliper pistons are around 2.5inch.The pads are kinda middle of the road. Theyre adequate from 60mph.Just.
-If your parts are smaller than this,then you will need a pretty aggressive pad to also be adequate, just. Or a more powerful mechanical advantage through pedal-ratio. Or a power booster. Or a gym membership.
-Playing with M/C bore sizes is great to fine tune the system, and personalize it. It cannot, however, compensate for a poorly designed mechanical advantage or a too-small system.
-Im not disrespecting the Scarebird system.I know nothing about it.
-And theres a wild card; weight. If your trunk is full of concrete, your gonna need a whole lotta brakes.
My 3450 # A-body has 15/16 m/c,KH 4-piston caliper system, 10x 2.5 rear system, rear w/c are 7/8, and a booster from a 73 Dart. Its the best braking car Ive ever owned. No competition driving.
-Hope this helps.