Blazer brake upgrade - Proportioning valve change required?

[Clarkwb100]

I upgraded the factory single piston brakes to the S-10 / Blazer dual piston assembly a couple years ago. The spindles were from a 2001 2wd Trailblazer, all the rotating assembly, the brake rotors, pads and brake lines were replaced at that time. The master cylinder was replaced with a stock 1987 El Camino unit prior to this major replacement.
I did NOT replace the proportioning valve.
Rear brake drums and pads were replaced with stock GM 1987 El Camino units.
The difference after replacing the single piston brakes was tremendous. But the dual piston brakes have always seemed "touchy". The front brakes lock up quickly now. Even at low speed in parking lots they will lock up if not careful. Inspection of the rear linings shows them working.
How do I get rid of this "touchiness". Replace the proportioning valve? Is this the fundamental problem? What brand & part # is recommended?

Your thoughts and guidance is appreciated.

 

[Built6spdMCSS]

When I built my SS in late 2010 I installed the front Blazer Brakes, had no issues with them.

 

[Clarkwb100]

My brake touchiness was corrected by replacing the almost new 1987 spec master cylinder with a 1978 master cylinder. problem was indeed the 1987 spec "quick take up" vs the older style. Stops great now with no low speed (or high speed for that matter) lock up of the front brakes.

 

[Clarkwb100]

I am changing out the "quick take up master cylinder" next week. I'll let you know if that was the issue.

Excellent advice. Changed "87 spec" quick take up MC to older style "78 MC". no lock up and working great!