Some confusion.

Hydroboost brake booster uses power steering pressure as assist. http://littlekeylime.com/MrN/img/hydroboost/hydroboost.png
Hydrovac uses vacuum as the assist. http://dave78chieftain.zxq.net/Hydrovac/HydrovacOperation.jpg

Old trucks may have had hydrovac boosters, and new ones including most all diesel pickups commonly use hydroboost.

•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-