2.2 Brake Booster - Bosch iBooster Gen 2 + Wilwood MC¶

Overview¶

Electromechanical brake booster required for the Cummins R2.8 diesel (minimal manifold vacuum). The Honda Accord Hybrid donor delivers the same Bosch Gen 2 unit as the Tesla Model 3 with better DIY documentation and sourcing.

The factory master cylinder is discarded. The Back Bay Customs adapter mates a Wilwood Tandem Compact master (260-15542, 1.00" bore) directly to the iBooster — solving the vertical-firewall reservoir-angle problem and putting the hydraulics on Wilwood's remote-reservoir ecosystem.

Vendor compatibility (Back Bay Customs / Adam, email 2026-05-30):

• The booster's 60×80mm firewall pattern¹¹ must be mounted 80mm vertical. Back Bay's 80mm-horizontal adapter is out of stock — design around 80mm-vertical.

Specifications¶

iBooster¶

• Current: 40A peak (braking), 0.25A idle, 12mA standby
• Mounting Bolt Torque: 13 Nm (115 in-lb) - 2× nyloc nuts, 13mm (M8)³

Master Cylinder¶

Wilwood 260-15542, 1.00" bore — sized to the TJ Rubicon front calipers and the exact part Back Bay confirmed the adapter against. Ready to order.⁴

• Bore: 1.00" — Wilwood 260-15542⁴
• Stroke: 1.10"⁵
• Outlets: tandem (independent front/rear); thread 1/2-20 IF⁶
• Reservoir/inlet ports: remote feed; thread is an in-hand check, not a datasheet lookup⁶

Brake Hardware (downstream — MC bore design inputs)¶

• Front calipers: Jeep TJ Rubicon — single-piston floating, 2.595" piston (~5.29 sq in each; ~10.6 sq in front total)⁷
• Rear: disc — factory TJ rear disc, PowerStop drilled & slotted rotors⁷
• Pedal: 03-06 TJ/LJ auto pedal (ratio bench-measured on assembly)
• These set the MC bore selection — see the Master Cylinder section and ⁴.

Plumbing¶

Reservoir → MC feed (low pressure): -3 AN PTFE braided — Wilwood 220-12993 (see Master Cylinder section). Gravity feed only; not a pressure circuit.

MC → calipers (high pressure) — hybrid hardline + axle flex. Baseline design; switchable to a more standard (or full-PTFE) approach later without changing the MC or its 1/2-20 IF outlets.

• Chassis runs: 3/16" copper-nickel (NiCopp / Cunifer) hardline off the MC's 1/2-20 IF outlets, 45° double (bubble) flare with inverted-flare fittings. DOT-approved for hydraulic brakes, will not corrode, and hand-bends/flares without a hydraulic bender.⁸ Copper-nickel is softer than steel, so sleeve in stainless spring "gravel guard" armor at exposed points and route inside the frame rail where possible.⁹
• Axle ends: DOT-compliant (FMVSS 106) braided-stainless flex hose at each axle, extended length sized to full suspension droop. Lengths are blocked on axle + suspension selection (Section 10 TBDs) — do not cut to length until ride height/articulation is fixed.¹⁰
• Hardline → flex transition: bulkhead fittings at each axle.
• Proportioning: the iBooster has no integral proportioning valve. Front disc + rear disc warrants an adjustable proportioning valve on the rear circuit (plus residual-pressure valves if needed); size on assembly. TBD.

Alternative considered — full -3 AN PTFE braided throughout: field-repairable and common on non-street comp rigs, but not chosen for a street LJ — FMVSS 106 certification attaches to the finished assembly (DIY bulk-hose builds aren't road-certified) and the braid hides the liner from inspection.¹⁰ The hybrid keeps inspectable hardline on the long runs and isolates flex to the axles.

Parts List¶

Wiring¶

Wire Transition: PMU terminals accept max 12 AWG. Two 12 AWG wires from OUT1 and OUT10 splice into a single 10 AWG wire near the PMU to minimize voltage drop.

See PMU Outputs for complete PMU configuration and thermal analysis.

Firewall Backing Plate¶

3/16" A36 steel sandwich plate that sits on the cabin side of the firewall, opposite the iBooster's integral 4-stud flange. Spreads cantilevered load across a larger area of firewall sheet metal to prevent fatigue cracking at the new M8 holes.

Geometry:

• 152mm × 152mm (6"×6") square, 12mm corner radius
• 4× 9mm M8 clearance holes on a 60mm (horizontal) × 80mm (vertical) rectangular pattern (80mm dimension vertical, per vendor)
• 64mm center pass-through bore (clearance for iBooster body neck protrusion)
• 3/16" (4.76mm) thickness
• Material: A36 mild steel
• Finish: zinc yellow plating (corrosion resistance for the cabin-side mounting area; also lets the plate stay visible-looking-intentional behind the dash)

CAD file: lj-ibooster-backing-plate.dxf (Fusion 360 — needs update to 60×80mm pattern)

Fabrication workflow:

1. 3D-print a 1:1 PLA prototype first for trial fit against the LJ firewall + iBooster assembly. Use this to confirm hole spacing matches the donor unit's actual studs and that the 64mm bore clears the booster body neck.
2. Verify clearance for cabin-side hardware (pedal box, HVAC blower, dash brackets) before committing to steel.
3. If PLA mockup fits, upload the DXF to SendCutSend and order in 3/16" A36 steel with zinc yellow plating finish.

Brake Pedal Assembly¶

The factory LJ manual-trans pedal must be replaced with the 03-06 TJ/LJ automatic brake pedal to remove the clutch pedal and provide the correct pedal arm shape for an iBooster pushrod connection.

Signal/wiring impact: None new — the stop-lamp switch (Mopar 56045043AB) is identical on manual and auto brackets. The pedal swap inherits the existing brake-signal wiring already designed across three systems:

flowchart LR
    SUPPLY["Switched 12V<br/>(accessory feed)"]
    SWITCH["Stop-lamp switch<br/>Mopar 56045043AB<br/>(closes when pedal pressed)"]
    PMU["PMU In 2<br/>via Deutsch Pin 13"]
    LIGHTS["PMU OUT21 (7A)<br/>Brake lights"]
    STARTER["Crank chain tap<br/>via Firewall Pin 15"]
    TCU["Turbolamik TCU<br/>brake input"]

    SUPPLY --> SWITCH
    SWITCH -->|"Pole 1 tap"| PMU
    SWITCH -->|"Pole 1 tap"| STARTER
    SWITCH -->|"Pole 1 tap"| TCU
    PMU -->|"In 2 closes → OUT21 ON"| LIGHTS

    style SUPPLY fill:#ffd93d,color:#000
    style SWITCH fill:#d1d5db,color:#000
    style PMU fill:#a5d8ff,color:#000
    style LIGHTS fill:#d1d5db,color:#000
    style STARTER fill:#d1d5db,color:#000
    style TCU fill:#d1d5db,color:#000

• PMU is sensor + brain, not load path: PMU In 2 senses pedal-pressed; PMU OUT21 (7A) carries the actual brake-light current. Switch itself only handles logic-level (<1A).
• Single pole used: TJ/LJ 03-06 stop-lamp switches are 2-pole (4 pins). Pole 1 routes to PMU/starter/TCU; Pole 2 was the factory cruise-deactivation pole — leave disconnected, not in this design.
• Splice location: Pole 1 output splices in the cabin into three branches — one to each downstream consumer.

See tail/brake (PMU lighting flow), starter (crank chain), and transmission (TCU brake input) for the downstream details.

Mechanical impact:

• Pushrod interface: Same on both pedal styles - iBooster clevis attaches at factory pushrod hole. Pivot-to-pushrod distance is identical; pedal arm shape differs (straight on auto vs. offset on manual) but does not affect the pedal ratio. Bench-measure before final assembly to confirm.
• BTSI provision: Auto pedal has a second boss for the factory floor-shifter BTSI solenoid - leave empty; Kilduff/Turbolamik handles unlock-from-Park electronically.
• Clutch removal: Clutch pedal, clutch master cylinder, and CMC firewall pass-through are no longer used. Plug the ~1.25" CMC hole in the firewall with a block-off plate or weld closed.
• Pedal pad: Auto pedal pad is wider - cosmetic only.

Donor checks:

• Stop-lamp switch + retaining clip present (or buy new Mopar 56045043AB)
• Pushrod hole bushing in good shape
• Pedal arm not bent/cracked at pivot

Installation Notes¶

• Firewall mounting strategy: Factory LJ vacuum booster holes are abandoned. The Honda Gen 2 iBooster bolts directly to the firewall via its own integral 4-stud flange (60×80mm M8 pattern, 80mm dimension vertical per Back Bay Customs; 62mm body neck through the firewall) — no separate steel bracket. Drill matching holes in the LJ firewall, then sandwich a SendCutSend 3/16" steel backing plate on the cabin side to distribute the cantilevered load and prevent fatigue cracking at the new holes. If an 80mm-horizontal orientation is ever required, Back Bay sells a re-orientation adapter (currently out of stock).
• Fallback paths: If integral flange has engine-bay packaging conflicts, fall back to (a) SendCutSend custom one-off firewall plate that re-patterns mounting holes, or (b) Back Bay Customs one-off commission — only worthwhile if they have access to an LJ for trial fit; without one, customer-measurement-blind design carries the same risk as SendCutSend at higher cost.
• Reservoir height: Mount reservoirs 4-6" above MC flange on firewall standoff for proper gravity feed. More than 6" risks hood clearance issues.
• Bench test before fab: Apply 12V ignition signal to the iBooster and verify motor cycles + pedal rod assists. Used iBoosters fail frequently.
• Cantilever support: Gen 2 is lighter than Gen 1 but on a vertical firewall through-hole the cantilevered load + Jeep vibration warrants a secondary front support point (inner fender or frame brace).
• Pushrod length (MC side): Back Bay's adapter pre-sets MC pushrod length - no adjustment required.
• Pushrod length (pedal side): Re-measure after auto-pedal swap; adjust per iBooster install kit instructions (Tulay's or EVcreate) before bleeding.

Installation Resources¶

• Wiring the iBooster - EVcreate
• Installing the iBooster - EVcreate
• iBooster Donor Vehicles - EVcreate
• Back Bay Customs Shop

Outstanding Items¶

Waiting on external action:

• ~~Confirm Back Bay Customs MC adapter compatibility with Wilwood 260-15542~~ — ✅ confirmed by vendor (Adam, 2026-05-30): adapter fits the 260-15542, is Honda-iBooster-only (not Tesla), and the remote reservoirs are fine. Blocker cleared.
• ~~Close iBooster eBay offer ($195 sent on $254 listing)~~ — ✅ offer accepted 2026-05-30 (listing #397546491129); part #s 46680-T3Z-A00 + 01469-TWA-A58 confirmed on listing

MC bore — resolved:

• ~~Recalculate the required MC bore from the brake-system hydraulics~~ — ✅ 1.00" → Wilwood 260-15542 (2026-05-30). Sized to TJ Rubicon front calipers + rear disc + standard/light pedal feel; factory-matched and agrees with the already-confirmed adapter fitment. See Master Cylinder section.
• Bench-measure TJ auto-pedal ratio on assembly — geometry/travel check only: confirm the ratio leaves MC stroke headroom (no long/sinking pedal) and the iBooster input rod gets full travel + full return. Does not affect the bore; pedal feel is drive-and-decide, not bench-tuned.

Backing plate redesign (vendor revised the firewall pattern):

• 🔴 Redesign lj-ibooster-backing-plate.dxf from the 72×72mm square pattern to the confirmed 60×80mm rectangular pattern (holes at ±30mm H / ±40mm V, 80mm vertical). Center bore (Ø64), plate size (152×152mm), and corner radius unchanged. Blocks the SendCutSend steel order.
• Update firewall drilling jig/template to the 60×80mm pattern before mocking up

In progress:

• ~~Source 03-06 TJ/LJ automatic brake pedal assembly~~ — ordered; verify donor checks (switch + clip, pushrod hole bushing, pedal arm condition) on arrival
• 3D-print PLA prototype of the revised backing plate (DXF) for trial fit; verify the 60×80mm hole spacing against donor studs and clearance for cabin-side hardware

Pending donor arrival (purchased 2026-05-30, listing #397546491129):

• Confirm casting part #s on the actual unit match the listing (46680-T3Z-A00 booster / 01469-TWA-A58 MC kit)
• Measure body-neck Ø against the 64mm backing-plate bore
• Bench test donor iBooster before any fab work (12V to ignition signal → motor cycles + pedal rod assists)
• Verify 4 mounting studs intact and body neck undamaged
• Photograph back face — finalize backing plate alignment + harness pigtail condition
• Select wiring harness: Tulay's Gen 2 universal (plug-and-play) vs EVcreate Gen 2 connector kit (DIY)

Pending mockup:

• Mock up iBooster + Wilwood MC assembly against LJ firewall (cardboard/3D-print) to verify engine-bay clearance before drilling
• Confirm flexline length after firewall mockup (220-12993 = 8", longer SKUs available if needed)
• Design / select reservoir standoff bracket location and height (4-6" above MC flange)

Ordering:

• ✅ Order master cylinder — Wilwood 260-15542 (1.00" bore) — bore resolved, ready to order
• Order plumbing: 2× 260-16392 reservoirs + 250-16393 dual bracket + 2× 220-12993 flexlines (vendor-confirmed)
• Order Back Bay Customs Wilwood MC adapter (confirmed vs 260-15542 — matches selected bore)
• Verify the 260-15542 reservoir-feed port thread on the actual MC in hand before buying any extra inlet fittings — Wilwood's datasheet leaves the inlet field blank (it's an integral-reservoir unit), so this is an in-hand check, not a datasheet lookup. Back Bay confirmed the remote reservoirs work with the adapter (Adam, 2026-05-30) and the 220-12993 is the flexline kit described to them.

Brake plumbing — caliper side (hybrid: copper-nickel hardline + axle flex):

• Order 3/16" copper-nickel hardline + inverted-flare fittings (1/2-20 IF at MC outlets) + stainless spring gravel-guard armor
• 🔵 Select extended-length DOT (FMVSS 106) braided flex hoses — blocked on axle + suspension selection (Section 10); size to full droop, do not cut until ride height is fixed
• Select adjustable proportioning valve for the rear circuit (iBooster has no integral proportioning) + any residual-pressure valves; size on assembly
• Plan routing: chassis runs inside the frame rail, bulkhead fittings at each axle for the hardline→flex transition

Fab + install:

• After PLA fit confirms: order SendCutSend backing plate in 3/16" A36 steel with zinc yellow plating
• Drill LJ firewall: 4× M8 clearance holes (60×80mm pattern, 80mm vertical) + 62mm center bore for booster shaft pass-through
• Plug clutch master cylinder firewall hole (~1.25" block-off plate or weld closure)

Related Documentation¶

• PMU Outputs - OUT1+10 and OUT19 configuration
• Engine Bay Ground Bus - Stud 7 ground connection
• Firewall Ingress - Mounting and wire routing