1996 Astro front brake won't release

Hi, working on a friend's 1996 Astro that the brakes drag after stopping. Both front calipers were replaced and it didn't solve it so a garage switched the hydro boost and master cylinder and neither of them solved the problem either. We checked each side of the front seperately today and right side works fine but the left front doesn't release. We switched out that caliper again today with another rebuilt one just to see if it was hanging up and the new one acts the same. One thing we thought was strange was that when we disconnected the brake line, fluid didn't drip out freely like we assumed it would when hanging there even though the reservoir is full. Before replacing anything else I wanted to see if anyone had any suggestions. Our next thought was to replace the brake lines but wondered if there was any other thing that was a common problem to cause this or if it could be something as simple as the brake line. It does work fine when applying the brakes but just won't return so we're wondering if the brake line doesn't want to allow the fluid to return back when letting off the pedal. Any input would be appreciated. Thanks.

I've seen this 3 times on my cars -- replace the rubber hose from caliper to the frame---they get laminated inside and act as a check valve- allowing only flow in one direction- and causing the caliper not to release- the test is to get it where it is locked up-- then crack open the bleeder- and see if it immediately releases- if so the hose is bad-- the metal lines can't be a problem- unless you have pinched one shut somehow.............

Thanks for the input. Just checking for any other ideas. Both front hoses were replaced yesterday and the left front still hangs up. Thanks for the help.

maybe blow out the frt lines with brake cleaner and refill again . check to see if they free flow though the metal lines also.