I’m baffled by this issue with my 2018 Colorado ZR2 2.8 LWN, which has 250k km. The truck still has the original front brake pads at 60%, but I’ve replaced the rear brakes three times already. I’ve had poor braking since day one. I assumed it was the master cylinder, so I replaced that along with the booster (not a fun job). I bled the system, bled the ABS, and then re-bled the system again. The pedal is firm when the vehicle is off, but when I turn it on, the pedal goes to the floor with virtually zero resistance. The brakes hardly apply enough to stop the vehicle. I’ve checked all the brake lines, calipers, and connections with no signs of leaks. The new MC was properly bench bled, and there are no lights or error messages on the dash. I don’t know what else to try! Thoughts?
I know there’s an ABS modulator that needs to be bled on Silverado trucks, and it’s likely the same for Colorados too. Have you checked that?
Sounds frustrating! Have you considered checking the brake proportioning valve? It might be affecting your rear brake performance.
Also, make sure you’re using the right brake fluid. Mixing different types can cause issues.
Did you check the vacuum lines? If there’s a leak, it could affect the brake booster’s performance.