Brake proportioning valve bad?

[madmaxp01]

I've got a frustrating problem. I wonder if anyone could offer me some advice.

I own a '67 CST10 with a 327ci, and drum brakes all around.

As I was leaving my driveway, I felt the brake pedal sink to the floor.
I discovered the short rubber brake line going to my passenger side brake had burst.

No problem. I changed the line.

As my friend worked the pedal to help me bleed the lines, I noticed very little fluid coming from my front bleed screws (less than from my rear bleeders). After a long time of bleeding, I started the truck to test the brake power. There was hardly any pressure under the pedal. It would travel all the way to the floor board. It would have been suicidal to drive it.

Both my master cylinder and booster are under 4 months old.

Is it possible that the proportioner valve could have blown as the brake hose burst?

Any advice would be much appreciated. Thanks!