The culprit seems to be a pressure accumulator used to seal a metal housing containing a metal plunger that transfers pressure to the brake fluid, separating it from nitrogen used in the rest of the system. The faulty plunger could develop fatigue cracks and allow nitrogen to leak into the brake fluid, thus increasing pedal travel and reducing braking force.
Luckily, it is not as dangerous as it sounds. The car should be able to detect the fault and activate a warning light. Toyota says affected cars should not be driven and drivers should get in touch with their dealers if they see the warning light. Only two such faults have been reported in the UK so far, so most users will probably have it sorted out before it comes to that.
On a side note, Toyota’s biggest and perhaps most embarrassing recall was caused by faulty accelerator pedals, which were blamed for a few spectacular and highly publicised crashes in the US a few years back. Now it’s the brake pedal, which only leaves one other other pedal to cause a massive recall sometime in the future, and that’s progress in our book.