Delayed Disc Brake Release

This is for sure not normal. You need to think "end to end." On the big pattern 73/4 brake conversions, there's a photo and paragraph in the manual of FITTING THE PADS to the caliper. This is rarely mentioned.

In no order
Sticking pads or other binding of caliper or pad fit, or even TAPERED DISK ROTORS

Hose problems
Problems in the dist block/ warning switch/ metering/ proportioning valve, AND I WOULD SPEND SOME TIME HERE

Master cylinder / boooster problems

Just about the first thing I'd do is get a helper and get "ready" so you can do this fast. Apply the brakes and confirm the problem, AKA attempt to rotate a jacked up tire. Quickly crack the line nut at the master and see if there is retained pressure. Move from there to the metering/ proportioning valve and crack both sides.

If you can, "end up" at the bleeder screw. If pressure shows up "at the wheel" but not on the master side of the proportioning valve, that may tell you something.