There is a picture of a master cylinder in the ad..
If looking for a donor for that sort of stuff there are plenty of options like it in various newer and more common trucks. First research if replacement MC and booster (from whatever you're looking at) can be had reasonably at the parts store to avoid swapping in expensive/obsolete parts.

•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-