Is it cause of the minimal amount of fluid or bad seal design? I seldom mess with my MX bike's brakes, and never bother to check my truck,...
But if you turn you truck upside down and pump the brakes they won't work, neither will the moto's. They all have an "open resivoir" but the bike's are still under a sealed bladder. Not nearly as much fluid or room for expansion. They simply need more frequent fluid change because of this.