Tim,
Most any truck parts place can has or get a functional replacement, as the same idea was still used for many years. Some of the systems actually had a dual circuit master and two separate "Hydrovac" units.

Finding an original one in new or rebuilt condition at a cheap price is somewhat harder to do.

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-