Rear Drum Brakes Locked Up

[65 Fastback]

Hi all, I have a problem I'm hoping someone can help me with.

My 72 C10 has always had a heavy Brake Pedal with little travel, i assumed it was normal not owning a truck before. Drums rear, Discs front.

Since i put in a rebuilt engine it has been even harder with less travel, i guessed the new motor was producing less vacuum.

Today i did an emergency stop and really put the boot into the pedal. The rear tyres didn't lock up, but when i went to move again it was obvious the brakes were not releasing. I was able to get it home and I jacked up the rear, put it in neutral and was unable to spin either wheel. I could however spin both front wheels easily. When i looked underneath the main hose to the T Block going to each rear drum was blown.

2 Hours later i could spin the rear wheels.

Im guessing the hose blew because of the pressure i put in the line when i stomped on it and it didn't bleed back to the master cylinder?

Is there an adjustment i need to make on the master cylinder/booster that would solve the issue of hard pedal and minimal travel?

Any insights appreciated. Pics below. Oh yeah and what the clamp on the rear brake line in the pic? is this some kind of budget proportioning valve?

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