I had my mechanic friend do basic maintenance and when the brakes wouldn't bleed properly I brought it in to be inspected. They found that the master cylinder had a leak and needed to be replaced, so I paid out of pocket for that. They signed off on it but said that the brake error light had tripped because my proportioning valve had stuck, but was assured that it may unstick itself if I drive it as normal. I did so and the brakes gave out, then locked up. My mechanic friend examined it, found both rear cylinders were stuck pressurized and replaced them both. The new ones immediately repressurized as well. We gave up and sent it back to the shop where they said "yeah, the proportioning valve balances pressure between front and back brakes, we told you it was stuck". Note that they told me to drive on it anyway, and neglected to tell me the function of the valve other than "it trips the error light". Cue them refusing to source the part because it is too old, and me having to provide an unwarrantied part and pay them $500 to install it.