Hard pedal with engine running, normal pedal with engine off = booster or vacuum supply failure. Vacuum check valve in the booster line is the common failure.
Brake booster failure on the XJ typically traces to one of three causes. 1) Failed vacuum check valve at the booster - the small plastic check valve in the vacuum line where it enters the booster fails open, allowing vacuum to bleed off at engine shutdown. Test: with engine off after sitting overnight, the brake pedal should give you 1-2 firm assisted presses before going hard. If the very first press is rock-hard, the check valve has failed (Mopar 56042377). 2) Cracked vacuum supply hose from intake manifold to booster - inspect for splits, especially at the elbows. 3) Failed booster diaphragm - this is the actual booster going bad. Test: with engine off, pump the pedal 5-10 times to bleed vacuum, hold pressure on pedal, then start the engine. The pedal should drop noticeably as vacuum boost kicks in. If it does not move, the booster diaphragm is torn. Replacement requires removing the master cylinder, dash bracket, and four nuts inside the cabin under the dash.
| Part | Vendor | Est. price |
|---|---|---|
| Vacuum check valve | Mopar | ~$15 |
| Brake booster reman | Cardone/RockAuto | ~$150 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.