Rear discs getting hot

Got an odd one. After a ride I can smell brake pads cooking. Splashed a handful of water onto the rotors and the rear boiled on contact - fronts are cool. I can put the backend in the air though and the tires freewheel in neutral as expected. The parking brake is inside the rotor hat and that looks fine too. Sure enough though, after every ride it gets warm again. Just hot enough to smell it and evap water. I've taken both sides apart can't find anything abnormal. Brake pedal has good feel and stops normally.

I don't know if this started happening before or after, but I had a line pop on the passenger front. Fixed the line, bled it and it functions as it should with the exception of this issue.

The setup is a Wilwood 4 piston caliper up front, rears are Explorer disc brakes using a factory dual reservoir master cylinder (don't know piston diameter). All the flex lines are new.

Any ideas? Thanks.

Pistons probably need to be manually retracted using a special tool. I've run into that problem before.

i the rear hanging free or is it up in it's normal position with the back of the car in the air?

Might be something hanging up with the axle at ride height and not up in the air? Ebrake cable maybe?

Has the RPV (if equipped) been removed from the rear circuit?

This is logical. And often overlooked.

Also suspect is booster rod adjustment.

Over the years I have seen a couple boosters that wouldn't retract fully but that's very unusual.

Yeah, swapped it out for a distribution block when I did conversion.

Got the stands under the axle. Actually never had the rear springs unloaded during diagnosis. I checked the hose going to the T and there aren't any kinks.

I'm not sure if these calipers do that. I'll give it a squeeze with a C clamp when I get back to the garage.

I didn't even check to see if the peddle comes all the way back up. Is it possible for it return after being stuck? Every time I get the car back into the air after returning home the brakes have already freed themselves.

I know you stated you have all new brake hoses, but I think I would replace the rear brake hose again. These days, being a new part no longer means its a good part. As far as the brake hose is concerned, you really have no idea how many years it has been sitting on a shelf waiting to be installed, or what may have been sitting across it during that time.

My bet is either on the brake hose, or the calipers. If either has a delay in releasing, that delay could create a lot of heat pretty fast in city driving where you use the brakes a lot in a short time span.

Jack the rear up and put it on stands. Have a helper get a feel for rotating the rear tires. Apply the brakes several times as you would normally while driving. Have them immediately check for drag on the wheels. if present pop a bleeder, see how much fluid comes out and if the drag goes away.
You either have pressure that is not releasing or calipers that are off center. once you establish which then you can pursue it.

You can tell if the rear calipers need the tool to turn them to retract by seeing if the face is smooth or has some sort of pattern in them. It could be little circular indents or a “C” that has extended ends. If they are the turn type and you crank on them with a c clamp, you can break them. I have done that in the past. If they at the turn type , try turning them out so the sides are exposed and lubricte them with some sort of anti seize. Then turn them back in and out a few times and see if they free up. They could just be full of dust. I would check to see if the rotors are out of round too or installed parallel so they don’t rub. You could also be warping them by overheating and hitting them with water. Also, is there enough free play in the flex hose when the suspension loads/ unloads? If the line hasn’t enough free play, it can send fluid into the caliper and cause them to squeeze without you knowing. If you do a high speed run and then let the vehicle coast, does it maintain speed and very slowly slow down or slow down more than you think it should? You could have the rears dragging while driving. Lastly, did you say both sides are doing this?

I had this problem with my Coronet but only on one side. I eventually just bought a new caliper for that side.

The RPV "Residual Pressure Valve" is often in the Master Cylinder in the port behind the brake line tube fitting.

Since you mentioned that you did the explorer rear brakes and the shoes in the hub look good then you should not have pistons that need to be retracted manually. The screw in pistons (manually retracted) are for the parking brake in the caliper assembly. Not the shoe in the hub type you have.

I have had new out of the box brake lines be bad and that would be the first thing I would check.

Since it is both the rear brakes it is probably the rear hose, the RPV, the proportioning valve, a kink/pinch in the brake line or something like that is causing the pressure "Stick" in the rear line.

If you can get a pressure gauge that will fit the bleeder port you can check to see if the front and rear pressures both drop properly and if you have a small amount of pressure causing the rear to drag.

The above is correct about the calipers.

My reference to the booster sticking is the pushrod, not the pedal side..I never got into the booster to figure out why they didn't retract fully, just replaced them.

To help narrow it down, go for a ride and when things act up, Crack the lines loose at the master. If there's a squirter then you're looking at master, booster, pedal. If not then move downstream.