Since it did it before the addition of the brake booster, there are a couple of other things that you can verify.
Of course, I'm leaning to a bad pump too, but what about the cooler? Is it in the pressure or return side? Should be in the return of course, and typically they are, but figured I'd ask because that would certainly add restrictions.
Any additional 90's? I don't think this is too often an issue or everyone would have trouble with their hydroboost. As far as I know most of them utilize 90 degree fittings in their plumbing and don't suffer this fate.
Even the custom made hose can have a restriction. This happens a lot in fact, with both hydraulic (steering and brakes), and fuel, and A/C hoses.
So new or not, custom made by a reputable shop or not, this can still be an issue.
Debris that is working it's way around the system can do it as well.
A filter might help determine if this is the case, and/or clean it up anyway even if it is not the problem.
And last for the moment, the quality of the fluid can have an effect. This is not as common, but maybe yours is somehow deteriorating when hot?
Paul