I can't loosen a caliper bleeder valve.

have you checked the rear brake rotors/drums for being warped ??
normally vibrations while braking are.. through the steering wh = front
thruogh brake pedal/seat = rear

good luck

Did you bleed the lines ? and also make sure there is no air goes in to lines.

If no fluid is "leaking" from the brake lines (inspect visually) then the brakes should be bled with this method: Use 1/8 inch 2 foot long hose from parts store and attach the hose from bleeder valve ball on the brake caliper to a bottle partially filled with brake fluid. The end of the hose in the bottle must be submerged in some clean brake fluid in the bottle. With a second person holding the brake pedal to the floor, slowly turn bleeder valve counter clockwise (loosen tiny bit) with wrench and watch for dirty brake fluid to create bubbles in the bottle. When bubbles start, quickly tighten bleeder valve back to tight. Second person should lift foot, pump and then while valve is still tight should push the pedal to the floor again and THEN you again open bleeder valve slowly turning counter clockwise and watch for bubbles. Repeat until no more bubbles can be created in the bottle where the hose is submerged. Move to next caliper and repeat. For a firm brake pedal you will need to repeat this on all four calipers.

The rear brakes on the Spectra will always engage less than the front brakes. If you have to push hard on the brake pedal then you probably have air in the brake lines from the last brake pad replacement. The brakes need to be bled with this method: Use 1/8 inch 2 foot long hose from parts store and attach from bleeder valve ball on the brake caliper to a bottle partially filled with brake fluid. End of hose should be submerged in bottle. With second person holding brake pedal to floor, slowly turn bleeder valve counter clockwise (loosen tiny bit) with wrench and watch for dirty brake fluid to create bubbles in the bottle. When bubbles start, quickly tighten bleeder valve back to tight. Second person lifts foot while valve is tight and then pushes pedal to floor again and THEN you again open bleeder valve slowly turning counter clockwise and watch for bubbles. Repeat until no more bubbles can be created in the bottle where the hose is submerged. Move to next caliper and repeat. For a firm brake pedal you will need to repeat this on all four calipers. My 2003 Kia's rear brake pads "fell apart" and never wore out in 190000 miles because they do not get used. The braking power is primarily in the front disk brakes on this model. Removing and replacing the rear drum brake linings did not even require me to bleed the brake lines. They weren't that tight before or after the replacement procedure.

If you have a soft brake pedal and have to push hard to slow down or pump the brakes to slow down then you probably have air in the brake lines from the last brake pad replacement. The brakes need to be bled with this method: Use 1/8 inch 2 foot long hose from parts store and attach the hose from bleeder valve ball on the brake caliper to a bottle partially filled with brake fluid. The end of the hose in the bottle must be submerged in the fluid in the bottle. With a second person holding the brake pedal to the floor, slowly turn bleeder valve counter clockwise (loosen tiny bit) with wrench and watch for dirty brake fluid to create bubbles in the bottle. When bubbles start, quickly tighten bleeder valve back to tight. Second person should lift foot, pump and then while valve is still tight should push the pedal to the floor again and THEN you again open bleeder valve slowly turning counter clockwise and watch for bubbles. Repeat until no more bubbles can be created in the bottle where the hose is submerged. Move to next caliper and repeat. For a firm brake pedal you will need to repeat this on all four calipers.

JACK UP REMOVE WHEEL BRAKE HOSE WILL ATTACH TO INNER FRAME RAIL AT SOME POINT LOOSEN THAT END WITH LINE WRENCH AND REMOVE LOOSEN END AT CALIPER INSTALL NEW HOSE DON,T FORGET COPPER WASHER AT THE CALIPER END TIGHTEN UP REMOVE BRAKE FLUID TOP AND FILL WITH FLUID HAVE SOMEONE PUMP BRAKES FOUR OR FIVE TIMES KEEPING PRESSURE ON PEDAL LAST TIME WHILE YOU CRACK UP BLEEDER SCREW ON CALIPER CONTINUE TILL NO AIR OR BUBBLES PRESENT TOP OFF MASTER CYLINDER

There is a bleeder screw at each caliper/wheel cylinder. You looosen this screw (don't remove it) and have someone depress brake pedal and hold it down. Tighten bleeder screw, release brake pedal, and repeat until you get only fluid (no air) from the bleeder.

Catch the fluid in something to kep it off the ground. Make sure you keep the master cylinder reservoir full of fluid during this process or you'll get air back in the lines.

You may have to do this several times for each wheel.

Try loosening the line at the master cylinder when they are tight. This is odd, usually the brake hoses would have fixed this problem. I don't see how a combination valve could cause this but let me know if popping the lines loose at the MC releases the brakes. It could have a bad master.

Try hitting the area around the bleeder with a hammer .. it may help .. bleeders are tough if they don't go after being heated/lubed. You could try loosening the line into the caliper .. that may allow enough of a bleed, banking on the fact that there is still brake fluid in that caliper .. otherwise you may end up replacing that caliper.

Try checking the caliper on the side that's wearing down faster. Make sure the caliper seal where the cup is does not have any damage. But it might be due to the fact that you only bled 3 out of 4 lines. Good Luck And PEACE