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Has anyone looked at the rear brakes during the diagnosis of this problem?

Rear disc or rear drum?

Does the park brake hold the vehicle from moving with it set and the transmission in drive; no application of the brake pedal?

In our shop, we see this quite a bit. More often than not, the rear brakes are not adjusted properly, and, if so, the rear brakes are not doing their share of the braking, which causes the front rotors to work harder to stop the vehicle; this creates more heat.

Also, a buildup of brake dust in the rear drums will compromise the rear braking ability

Does it move with the park brake on, in drive, whilst giving it gas?

Have you removed the rear drums and checked for brake dust?

That would be my next step; remove the rear drums, clean the interior of the drum with brake cleaner, sand the brake shoes with some 80 grit sandpaper, and then adjust the rear brakeshoes

Before you do that, raise the rear wheels off the ground and spin the tires. (park brake off). If the tire spins very freely, with no drag, the rear brakes need adjusted

There sgould be a tiny bit of drag when you spin the rear wheels, just a touch.

There have been known issues with the front brake hoses constricting, either with age or a previous caliper replacement where someone clamped vice grips on the brake hose to stop the brake fluid from leaking out. This constriction can keep some pressure on the front calipers

The pressure will be good. The constriction will keep the pressure applied, if there is an issue; this will not prevent fluid form coming out when you bleed.

Rear brakes: Raise the vehicle. Spin the rear wheels as fast as you can by hand. If they do not slow down in 2 or 3 rotations, the rear brakes are not adjusted. Click the starwheels until you hear a slight brake shoe rub on the drums as you spin the tire. Slight rub. Very slight,.

Think of the constriction like when you kink a garden hose just a bit. The water still comes out, but the pressure is different.

:-)

Sure. You do need a couple cans of spray brake cleaner to do the job right, and a piece of 80 grit sandpaper.

How many miles on the Sienna?

oh.

yes, I would consider replacing the front hoses.

I doubt it....perhaps the master cylinder is not developing proper pressure to the rear wheel cylinders, but that would be rare. Possible, but rare. What color is the brake fluid in the reservoir" Clear or dark (before you started)

That is good. Dark fluid = moisture in the system, which could indicate rusted wheel cylinders.

Brake fluid absorbs moisture. Always change it when you cannot see thru it

OK

Reddish?

Like red hot reddish?

And both of the front wheels spin freely?

anybody change the master cylinder?

Why did you change the pads the rotors and the calipers?

Does the master cylinder reservoir lid have a rubber gasket under it?

is it deformed, bigger than it should be?

and the brake rotors get red hot/glowing red?

You have some kind of issue keeping the brake pressure on. This could be a mis-adjusted brake master cylinder rod (it might be adjustable; back it out a few turns). The brake hose constriction is sounding like the issue, however. The brake rotors only get hot form fraiction, and the only way you are going to have friction enough to get the rotors red hot is either the calipers are staying applied, the hoses are keeping pressure applied, or the master cylinder is keeping pressure applied.

or your caliper slides are stuck, or you have the wrong pads and calipers installed

Some remahn caliper pistons stick in the bores. Do the pistons retract easily into the new calipers when they are hot?

At this moment, if the car was in my shop, I woudl be replacing the calipers and the brake hoses before I did anything else.

I would check that, and, if they are tight, replace them.

Ok

Please let me know how you make out