With an Easy-Out.
You're simply going to have to have it removed and replace it. That nipple is at the lowest point in the system and there is no other way to drain it unless you have the system power flushed.
Replace the caliper and bleed the brake system.
Replace the caliper and bleed the whole system
remove the caliper slider bolts and the brake hose then put the new caliper on, dont forget to bleed the new caliper and top off brake fluid when finished
Well after taking off the caliper, and have taken the old pads off. You are going to have to push the caliper cilinder back in with or with-out opening the bleeder ( the nipple on it). After that, you will see a small metal ring that moves around, your brake pad snaps onto that n that is what holds your inner pad into place, the outter pad slides n holds onto the outter part of the caliper. Replace n bleed if you need to.
When you bleed brakes it is necessary to bleed each caliper in turn.
Remove the front pad by realising the spring, then take the two bolts holding the caliper off clamp the flexi brake pipe and release the bleed nipple. remove the rear pad and wind back the piston either with the correct tool or a pair of pliers. fit both pads back into the caliper and return over the disk, tighten the bleed nipple and release the pipe Job Done
You will need to bleed the brakes, starting from the caliper furthese from the master cylinder then moving closer.
what is the best way to bleed brakes when installing new caliper on front left wheel
Since you did not give any details I will have to answer with some general suggestions. 1. standard caliper, use a C clamp to compress the caliper ( no need to bleed system ) 2. If you do not have a C clamp, release the pressure by turning the bleed screw and push the caliper in. you will need to bleed the system if you do this. 3. certain vehicles need a special tool that turns the caliper in ( will have a slot in the piston ), they are pretty cheap and your local auto parts store will have them. When you get there get all over the parts counter person for not mentioning that you would need one.
Remove the tire, and just unbolt the caliper, there are 2 bolts from the back. Now the caliper will just slide up and away from the rotor. If you have to replace the caliper, then check the pads and the rotor. Might have to replace them also. Be careful removing the brake line. Work it slowly so as not to break it. You have to bleed the new one. Take off the brake line first and hang it up to keep it clean.
Remove the tire then remove the brake caliper take the pads off and replace them just like you removed them. Put the brake caliper back on you will need to bleed the brake line on the caliper add brake fluid if needed and put the tire bak on.