The rear brakes seem unable to bleed. i think the proportioning valve bad.

Resetting a proportioning valve in a 2001 Ford Explorer typically involves bleeding the brake system to ensure proper brake pressure distribution. The proportioning valve adjusts the amount of brake pressure sent to the front and rear brakes to optimize braking performance. If you're experiencing brake-related issues or have replaced brake components, bleeding the brake system can help reset the proportioning valve. Here's how to do it:
**Tools and Materials Needed:**
- Brake fluid
- Jack and jack stands
- Wheel chocks
- Wrenches
- Brake bleeder kit or clear plastic tubing
**Steps:**
1. **Safety Precautions:**
- Park the vehicle on a level surface and engage the parking brake.
- Use wheel chocks to prevent the vehicle from rolling.
- Wear appropriate safety gear, including gloves and eye protection.
2. **Prepare the Vehicle:**
- Loosen the lug nuts on all four wheels, but do not remove them completely.
3. **Jack up the Vehicle:**
- Use a jack to lift the vehicle off the ground. Secure it with jack stands for safety.
4. **Bleeding Sequence:**
- Start with the brake caliper or wheel cylinder that is farthest from the master cylinder. In most cases, this would be the right rear wheel.
5. **Bleeding Process:**
- Have a helper pump the brake pedal several times and then hold it down.
- Loosen the bleeder valve on the brake caliper or wheel cylinder.
- Fluid and air bubbles should come out. Tighten the valve before your helper releases the brake pedal.
- Repeat this process (pumping, holding, opening, and closing the valve) until you see clear brake fluid without air bubbles.
6. **Repeat the Process:**
- Move to the next farthest brake caliper or wheel cylinder (left front, right front, left rear) and repeat the bleeding process.
7. **Check Brake Fluid Level:**
- Keep an eye on the brake fluid reservoir while bleeding. Make sure it doesn't run dry, as this can introduce air back into the system.
8. **Top Off Fluid:**
- After bleeding all the brake components, top off the brake fluid reservoir with fresh brake fluid to the recommended level.
9. **Test Brakes:**
- Lower the vehicle, tighten the lug nuts, and then fully tighten them in a crisscross pattern.
- Test the brakes to ensure they feel firm and responsive.
Bleeding the brake system is an important maintenance task, especially after working on the brakes or if you're experiencing spongy brakes. If you're unsure about the process or want to ensure proper brake system maintenance, consider seeking assistance from a professional mechanic.

To my knowledge, the porportioning valve can't be bled using the ABS bleed feature of a scan tool and has to be manually bled.

The brakes will need to be bled normally, and GM has a depressor tool specifically made to be installed on the valve while bleeding the brakes: J-39177.

You'll have to bleed the front and rear brakes as normal, but with the tool installed in order to remove the air trapped in the valve.

Proprtioning valve might be stopping fluid going too back wheels

None of those things . Just bleed the brakes normally starting from the longest line and ending with the shortest. The proportioning valve will always let brake fluid through, just not enough to lock up the rear brakes until you press heavy on the pedal . It is there so that full pressure to the large front brakes is reduce in proportion to the smaller rear brakes.

check for fluid before and after the proportioning valve by disconnection the line after the valve and applying the brakes, if no fluid after valve try before valve if fluid is there then it is a bad proportioning valve. you did bleed the master cylinder before connecting the brake line and trying to bleed the brakes all at once right? If the master cylinder was not bled first then a lot of air may have been forced into the rest of the brake lines.

All bleeding is done at each caliper and wheel cylinder. The proportioning valve has no bleeder. The proportioning valve makes the front brakes engage before the rears do to prevent the rear brakes from causing a sideways skid.

Blocked line; if there is fluid to the valve and none after it you need a new valve

sometimes you can tap on the proportioning valve while someone pumps the brake if nothing breaks loose you may have to replace the valve.

It could be a proportioning valve, and there's also the chance that the portion of the master cylinder that operates the front brakes is going bad. Have you had a pressure check done on the front brake system? Also, some vehicles have a variable proportioning valve, so that when the vehicle is loaded, it allows more pressure to the rear brakes. These can sometimes stick in place, causing the fault that you describe. Hope this helps you out.

The proportioning valve most definately can be the problem. Did you change it or is it the original? (A disc/drum system uses a 70%front/30%rear braking ratio, whereas an all-disc system is alot more even). I would first bleed the master cylinder. Then I would 'gravity bleed' the rest of the system by opening the bleeders on all 4 wheels. Do not at any point let the m/cylinder go dry. Also, do not touch the brake pedal.
Continuously check all 4 bleeders, when clean brake fluid with no bubbles at all is running from a bleeder, close it. When all 4 wheels are done, recheck the fluid level and you should be OK.
Pump the pedal to 'self-adjust' the rear calipers, then re-check the fluid level.
Any problem after that means you either have a loose connection (it's possible for a tiny leak to let air in, but not let fluid leak), a leak, or you need a new (disc/disc type) proportioning valve. I would try Summit Racing myself. Good Luck!