Manual drum to 4 wheel disk

[Crusty]

  finally after almost 4 years my Cuda is done except a problem with brakes. Switch every to factory style front power disk. New everything including proportioning valve set up. Upgraded to rear disk. Almost everything from inline tube. Now pedal is fine until u start the car then it almost goes to the floor and front brakes keep locking. Brake light also on when you push the pedal.. Rear brakes seem to be working fine. Got Another new master cylinder but that didn't help. Really driving me crazy because that is all keeping from driving finally.

[anlauto]

Beautiful looking car

[Crusty]

Thank you!

[Marty]

What a stunning looking 'Cuda! 

[Skdmark]

"Now pedal is fine until u start the car then it almost goes to the floor " sounds like a possible vacuum / booster issue.

Here are a couple of tests for the booster.
https://m.roadkillcustoms.com/test-power-brake-booster/

[Crusty]

Did all thoe tests to see if booster was bad. All good. now the front brakes stick till you tap pedal a second time and they release..weird as hell!!

[Bullitt-]

 What master cylinder are you using?   

[Crusty]

Stock power front disk brake one

[Bullitt-]

  FWIW I'll share my experiences although locking front brakes was not involved...
On both my '73 after rebuilding the front disks & rear drums and another member's '70 who had recently paid for a complete brake job the pedal would go to the floor with the engine running, but seemed normal with engine off. (He had just driven 300+ miles when his problem appeared) I had discovered that my rear drum adjusters were not engaging and sure enough when I checked his weren't either. Correcting this & getting the rears adjusted properly solved both cars issue...
May or may not apply to your rear disk conversion but the couple of rear disk cars I've dealt with had some system responsible for self adjusting.... 

[Crusty]

Ok thank you! I will check into it this evening.

[70vert]

I have had my pedal "stick" which means the brakes drag a bit, I had to put my toe under the pedal to pull it back to release it. Aligned, tightened, and lubed the linkage and problem solved.

SSBC 4-wheel disk with Willwood master cylinder and HydroBoost.