Brakes do not release

Funny. My cousin dumped his Sienna and is driving a Caravan now.

The first step is to actually diagnose the cause of this problem, not just throw parts at it. When this occurs again, park on a slight incline, place a block a few inches downhill of one tire, shift to "neutral", then open the hydraulic system in various places to see what lets the brakes release.

You had a dandy observation that a front brake and the opposite rear brake lock at the same time. Most front-wheel-drive vehicles use a split-diagonal hydraulic system that pairs the opposite corners in each of the two hydraulic systems. What worries me is that two brakes are locking at the same time. That tends to rule out a caliper in favor of what both brakes have in common. That's the master cylinder.

Where I would start with opening the hydraulic system is to loosen the steel lines right at the master cylinder. If that lets the brakes release, there's one of two causes. One is something happened to the bracket for the brake light switch and the switch is holding the brake pedal down a little. My worry is the second possibility. That's brake fluid contaminated with a petroleum product. That will cause the internal rubber lip seals to swell and grow past the fluid return ports, blocking them. As the brakes drag and heat up, that heat migrates into the brake fluid causing it to expand. Since it's trapped and can't return to the reservoir, the expanding fluid applies the brakes even harder, and more heat is generated.

If loosening the lines at the master cylinder don't let the brakes release, we can breathe a sigh of relief. Next would be to crawl underneath and open the bleeder screws on the calipers. If they still don't release, the best suspect is the mounting hardware has rust pits or a slide bolt is bent causing the caliper to not release properly.

Does your van have anti-lock brakes?

Thanks for the response!
Yes, I have antilock brakes.
I bought a proportioning valve, but haven't had time to change it out yet.
As to contaminated brake fluid, I'm pretty careful to not get any contaminants in it.

There shouldn't be a proportioning valve if the van has anti-lock brakes. The story is the same though. The goal is to get the brakes to lock again so you can open the hydraulic system in various places. That will help us determine where the brake fluid is being trapped.

They are stuck right now, but we are in the middle of a move, and time is one of those 4 letter words. I'll try to crack the lines at the master tomorrow. Thanks!

I'm here every day. Will be waiting to see what you find.