Some/many master cylinders come with residual pressure check valves built into them, in which case external ones would be unnecessary/redundant.
Then in some cases you don't need them any how.
I'd like to find a book on brake systems with all this explained... There is way to much rumor and misinformation floating around on the net about brake systems... just hope I'm not adding to it.

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-