2002 Jeep Cherokee 2.7T DR - P0657 Code: Brake Boost Issue Diagnosis & Vacuum System Troubleshooting
You may be on to something: When you mentioned the brake check valve, it could point toward a brake boost issue—specifically, a firm pedal without proper power assist. This vehicle features a turbocharged engine, and its brake booster relies on vacuum generated by a vacuum pump and associated valves and hoses. A failure in this system can result in poor braking performance, especially at low engine speeds. Before attempting to bleed the brakes, it's critical to diagnose the brake boost system. Using an OBD2 scanner during bleeding cycles can help—because scanners don't just display codes; they also control ABS valves and accumulators, which assist in properly bleeding the hydraulic brake lines. Key diagnostics include checking the vacuum level at the brake booster, verifying whether it holds vacuum when the engine is off, and inspecting the one-way valve in the boost system.
I've already inspected the brake booster hose from the front of the engine to the one-way valve—everything appears intact. I do have a spare vacuum pump on hand, and I haven't yet tested the one-way valve. The brake boost system was my first suspect when I noticed the issue. The brakes feel weak overall, but they become significantly worse when the engine is idling. Thanks for clarifying how an OBD2 scanner can assist during bleeding—it's something I wasn't aware of before.