Matrix Brake Booster Pressure Sensor Fault

Sounds like you've got a serious issue! A faulty brake booster pressure sensor definitely explains the error message and poor braking. If air gets trapped in the system, especially near the ABS module or brake booster, it can cause all sorts of weird problems. Air is compressible, unlike brake fluid, so it will affect the efficiency of the hydraulic brake system and in turn the sensor readings. Did you follow the proper bleeding sequence for your Matrix, and did you make sure to keep the master cylinder topped up?

I thought I followed the bleeding sequence correctly, starting with the wheel furthest from the master cylinder, and so on. But keeping the master cylinder topped up? I might have let it drop a little too low a couple of times. Could that REALLY be the culprit? And if so, is the sensor actually broken, or just giving a false reading because of the air?

Letting the master cylinder run dry during bleeding is a classic mistake, and yep, it can absolutely cause these issues! It introduces air into the ABS module, which is a pain to get out. As for whether the sensor is truly bad or just misreading, it's tough to say without more diagnostics. The sensor itself might be fine, but the air in the system is giving it a false reading. However, repeated false readings can damage sensitive electronic components over time. I would recommend taking it to a workshop and letting a professional sort it out. They have the tools and know-how to properly bleed the ABS and diagnose the sensor.

Thanks a million for the advice! I suspected I might have messed up that brake bleeding. I took it to the shop. Turns out, air in the system was indeed the main problem, but the pressure sensor was also damaged, maybe because of the bleeding. They replaced the sensor and re-bled the brakes correctly, costing me 90€. Lesson learned: I'm sticking to oil changes from now on!