Should I bleed my brakes after a piston popped out?

Q. I changed my brake pads today and when I pushed one of the pistons back another one popped out.

I managed to get it back in OK after a small spillage and it’s not leaking at the moment. Will I need to bleed the brakes?
davidstowe358, MCN forums
 
A. Yes, you will. If it’s not leaking you haven’t damaged the caliper seal with any luck, but if fluid has spilled out, there’s going to be air trapped in that caliper and the lever will fell spongey.

It won’t hurt to do both sides, especially if the brake fluid is old and brown.