Other likely cause is an internally collapsed rubber flex line acting as a check valve, you can't see the problem from the outside. I have run into that before, but never on a new flex line although it could happen..

•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-