Mini Cabrio brake pedal feels spongy

Low brake fluid could definitely explain the pedal feel. Is the level critically low, or just a bit under the 'max' line? Have you checked for any visible leaks around the brake lines, master cylinder, or at each of the wheels? It's possible you have worn pads, which would require more fluid in the system to compensate, but a leak is also a concern. When was the brake fluid last replaced?

Thanks for the quick response! The fluid level is noticeably low, well below the minimum line. I haven't spotted any obvious leaks, but I also don’t really know what I'm looking for to be honest. The brake fluid hasn't been replaced since I bought the car, and I don't see any mention of a fluid replacement in the service history. Does this mean a fluid replacement is the next logical step, or is there something else I should check before?

Given how low the brake fluid is and the age of the car, it's quite likely that the brake fluid is the culprit. The fluid attracts moisture over time, which reduces its effectiveness and can cause that spongy feeling. However, the bigger concern is where did the brake fluid go? Since you mention you don't see any leaks and that the fluid has never been replaced, a likely scenario is simply worn brake pads. As the pads wear, the caliper pistons extend further, requiring more fluid to fill the space. I'd still advise you to get it checked out at a workshop, as low brake fluid can compromise your car brakes.

Just wanted to give you an update. I took the Mini to a workshop today, and it turned out the brake fluid was indeed low, most likely due to worn car brakes. They did a fluid replacement and sorted everything out. The bill came to around 480€. Thanks for your help and advice!