[Moparteacher]

The seal on the caps must seal the fluid from the atmosphere. The vent in the cap directs atmospheric pressure to the top side of the seal.

Brake fluid is extremely hydroscopic in that it obsorbs moisture from the air, hence the seal.

If the fluid is pushing past the seal, and the seal is good, then the fluid may be contaminated. Moisture and oxygen in the fluid will expand with heat and push past the seal. Contamination will also lower the boiling point of the fluid.

Consider flushing the system and replacing with new fluid from a previously unopened container.