I have a Shimano BR-MT201 Hydraulic Disc Brake on my rear brake. I was replacing the brake pads and accidentally pressed the brake too many times and the piston popped out leaking brake fluid. Afterwards I pushed the pistons back in all the way and pumped new brake fluid into my brake but, when I press the brake the pistons don't move at all and the brake easily compresses all of the way. I also noticed a little air bubble around the piston when I press the brake. What should I do to fix this? I have also cleaned the piston with isopropyl.
This is applicable to hydraulic brakes on cars, trucks or bicycles because they all work on the same principle. When the piston came out air was introduced into the system. Air compresses easily, so when you apply the brake the force goes into compressing the air instead of pushing the non-compressible hydraulic fluid against the piston, and you get no braking action.
The solution is to bleed the brake, this pushes new fluid (and the air along with it) from the master cylinder (on the handlebar) to the caliper while not allowing air back in. With a car this is usually easy, if you don't have a specialized kit all you need is a piece of clear plastic tube, an empty plastic bottle, a bottle of brake fluid and a friend who will push on the brake pedal on demand.
With a bike it's the same process, but you are much better off buying a kit rather than trying to piece something together. I tried that, and realized I'd spend more money trying to source the individual components for the kit than buying it straight out, and the kits are pretty cheap. Just make sure the one you buy is right for your brake, it's pretty straightforward. There are good video tutorials on the web so I won't repeat their material.
You will have some air trapped in the brake pipes or brake cylinders. The brakes wont work properly until you get all of the air out, since the movement of the brake lever will just compress the air in the system rather than activating the brake. Since you are not seeing any movement of the brake piston, I would guess that you have a lot of air in there somewhere.
You can purchase brake bleeding kits for bicycles, for example -