I am trying to 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

you can loosen the bleeder valve and then use a C clamp to press it back in and then retighten the valve, You will lose a little fluid but will not need to re-bleed the lines.
Please vote if this helps

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.

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

worn out caliper maybe. is it on one side??? if so may be a bad brake line.

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 putting a six point socket with ratchet on the bleeder screw and try turning while tapping the top of the ratchet with a hammer.

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