Wrong master cylinder on the F 150 body is to blame. It was never for a 4 wheel disc system.
Try the late model F 350 master, if it was for 4 wheel disc. You might as well use the hydra boost also so you have assist to apply those binders.
A larger bore master cylinder will have enough volume to run the 4 wheel discs but may require a stronger assist to apply them to the full stopping capability.
Fact is stranger than Fiction