No pressure needed to return the shoes on the hydraulic brakes on your 55 6400.

When working the springs retract the shoes when you release the brake pedal. Just like normal drum brakes on most cars and trucks.

Likely the shoes are rusted to the drums.
Have you tried driving it, they may brake free if you try?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-