You could be developing a blockage in your brake lines. I had an Astro van that did that. I'd have to wait 5 or 10 seconds for the brakes to release. I replaced the rubber lines by the front discs and the problem was gone. The rubber likely deteriorated, and debris clogged the line, kinda like my arteries after eating a Big Mac. Good luck.
Sry I missed Diagnosed it was the rear brakes locking up, Found that the emergency brake assembly was missing and for some reason that was causing it to lock.I have since replaced the pieces in each side and so far so good