2006 honda lx. cannot depress either side rear brake piston.Tried releasing bleeder valve still cannot depress piston

If you replaced the front calipers. Make sure they went on the right side. Bleeder valves on top. A guy at the shop just did that. As far as the rear, if they don't release, replace the brake hose/s

The proportioning valve simply ensures that the front brakes are applied before the rear brakes are, to prevent sideways skids from loss of rear traction. The valve also serves as a warning light when one end, either front or rear, of the system has deficient line pressure. Inside the valve is a cylinder, and a piston that belongs in the center of the valve cylinder (when fluid pressure on each side is equal). When one side of the valve (front or rear brake system) loses pressure (like from a leaking brake line), the piston gets pushed to the low pressure end and makes contact with a switch terminal that turns the light on.The trick to turning the light off is to figure out which end, front or rear brakes, the piston is on, and then bleed the other end to center the piston and turn the light off. Too much pressure on the brake pedal will cause the piston to go past center to the opposite end, and a close watch on the brake warning light will reveal that as the lamp will flicker once while the piston passes center.
Gently depress the brake pedal (requires an assistant) with a bleeder open and close the bleeder as soon as the assistant (brake pumper) sees the light go out. If he sees the light flicker once bleed the other end of the vehicle to go back towards center.

open bleeder see if it releases. if it does , replace brake hose

Clockwise-on the rear
I would open the bleeder valve first, push piston in,then screw parking brake in.
You need to flush your brakes every couple years,anyway.
After you install new pads,ratchet brake up by hand.

If you have disk brakes, and you cannot fit the new brake pads in the callipers? Loosen the brake fluid reservoir cap to allow pressure to escape. Use a large C clamp connecting it to the front and back of the piston. Turn the screw on the clamp to compress the brake piston then release the clamp. The piston will remain compressed allowing you the fit the housing over the new brake pads.

If the car has anti lock brakes open the bleeder screw to allow pressure to escape as you use a C-clamp to depress to piston back into the caliper make sure to lock down bleeder after the caliper piston has been depressed.

Have you tried releasing the pressure within the brake cylinder. You will need to do this by loosening the bleeder valve or loosening the brake line going to the caliper.

Try turning again clockwise with a little more pressure. If the brakes were worn badly the piston is just stuck from being adjusted out so far.

Yes air gets into the hole system. You need to bleed all 4 wheels.

1 Siphon off the old brake fluid in the master cylinder reservoir under the hood.
2 Fill the reservoir with new brake fluid (don't reuse old fluid as it holds moisture and will corrode your brake system).

3 Start with the brake farthest from the Master cylinder, usually rear wheel, passenger side. Pull off the rubber cover on the bleeder valve. Attach the aquarium air line to the nipple on the bleeder.

4 Have your assistant pump the brake pedal up and down 3-4 times, then hold the pedal down fairly hard.
5 Open the bleeder, let fluid flow out through the air line into a waste container. (preferrably a clear bottle so you can see any air bubbles escaping the system). When the fluid stops and your assistant's foot hits the firewall (brake pedal all the way down), close the bleeder valve immediately!
6 Go to step #4 above and drain more fluid out of the bleeder valve until clean fluid comes out. Check the master cylinder every time and top it up as necessary. This procedure uses lots of fluid.

7 Now move to the rear brake on the driver's side. Go to step #4 and repeat the same procedures above until clean fluid comes out of the bleeder valve and no bubbles.
8 Next is the front brake, passenger side, same procedure. Then the drivers side and the same procedure.
9 When finished check for any leaks, remount rubber covers on the bleeder valve nipples. Remount all wheels, re-torque lug nuts after 50-100 miles of driving.
10 Start vehicle and depress the brake pedal. It should feel much less spongy than before this procedure. If not, there is air in the system, start over. Bleed everything again.
Note: if you assistant releases the brake pedal before you can close the bleeder valve then air will be introduced into the brake lines. This is no good! Before you start tell your assistant you will tell him/her PUMP, HOLD DOWN, RELEASE commands.

That's it, you're done. Congratulate yourself on saving some cash instead of handing it over to a mechanic.