My rear passenger side brake keeps locking up when I push even slightly on the pedal?

Opening the cap was just a coincidence. One of these things could be the problem that side may have sat for a while gotten wet and formed rust that could be the issue. Second may have sticking wheel cylinder.

The backyard mechanic would remove the brake drums and closely inspect the shoes, springs and if the parts seem to be in good shape then lubricate, adjust and retest.

The wheel cylinder may be defective because it has pistons at each end of it that move outwards to push the brake shoes into contact with the brake drum.When you let off the pedal,they release pressure from the brake shoe and go back into the wheel cylinder.The pistons might be sticking and not releasing.If the wheel cylinder is not defective,check the springs next on the brake shoes.The springs help pull the brake shoes away from the brake drum after you release the brake pedal.If the springs are worn,there is no other part on the brake system that can pull the shoes back into their normal position,away from the drum.If the springs and wheel cylinder are good and the brake line doesn't have any kinks in it,check the pressure differential valve/proportioning valve between the master cylinder and firewall or the hose that goes from a single brake hose and splits into the driver and passenger side rear brake hose.Make sure there's nothing wrong with those hoses and the fitting

Probably the only way to figure this out is to take the wheel off and inspect the brakes and related equipment. I doubt that bleeding the brakes would cause this to happen.

You need to have it inspected. If it done it once there is a good chance it will do it again. I had a car pop the lining and it wedged its self in the drum and locked up the wheel.

your rear break is not releasing so you need to get it looked at

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