It's just a cap (lid) there should be no pressure on the lid. Don't fill it to the top. when you press down on the brake pedal the fluid level will go down a little and when you let up on the pedal the fluid will return to the reservoir. The lid should just be "snugged" up. it may even have a tiny vent hole in it. Watch this YouTube video. you can go to the 23.00 min mark when he puts the top plate and cap on it.
https://youtu.be/igZgswGAvi8