XJ stock master cylinder bore is 15/16 in (pre-disc-conversion). ZJ 1 inch bore master is the common upgrade after rear disc conversion or with larger calipers.
Stock XJ master cylinder has a 15/16 inch bore designed for the OE disc/drum combo. After upgrading to rear disc brakes or installing larger front calipers (4-piston upgrades), the stock master cylinder lacks volume and produces a long, soft pedal. The 1993-1998 ZJ Grand Cherokee master cylinder has a 1 inch bore and bolts directly to the XJ booster - this is the standard upgrade path. Source: junkyard ZJ ($30-50) or new Cardone 13-2210 ($60). Replacement: bench-bleed the new master before installing (clamp in a vise with the front facing up, fill with fluid, push the piston gently with a screwdriver through several strokes until no bubbles emerge from the ports). Install with new gaskets, connect lines, and bleed the system normally. For race or trail builds with 4-piston front calipers, look at a 1-1/8 in bore master (Wilwood). Beyond a 1-1/8 in master you need to size the booster up too or pedal effort becomes excessive.
| Part | Vendor | Est. price |
|---|---|---|
| ZJ master cylinder (1 inch) | RockAuto | ~$65 |
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.