2517902C91 | Genuine International® Brake Master Cylinders

• International® DuraStar Series: 4300, 4400 medium-duty commercial trucks.

• International® 4000 Series: 4200, 4300, 4400 (2002–2010 production years).

• IC Bus® Chassis: CE Series, FE Series, and RE Series commercial / school buses equipped with hydraulic brakes.

• International® WorkStar Series: 7300, 7400 severe-duty applications fitted with hydraulic brake configurations.

1. What is it?

Part is the primary hydraulic pressure generator in the brake system. Mounted directly to the brake booster assembly, it converts mechanical force from the brake pedal into hydraulic fluid pressure, transferring energy through the brake lines to actuate the wheel calipers or wheel cylinders.

2. Core Functions & Technical Features

• Dual-Circuit Hydraulic Design: Split internal piston design isolates front and rear braking circuits, maintaining partial stopping power if a hydraulic leak occurs in one circuit.

• Cast Iron / Heavy-Duty Body: Engineered from high-strength cast metal to withstand high operational pressures without flexing or cracking under severe braking loads.

• Precision Bore & Seals: Precision-honed internal cylinder bore with high-grade EPDM seals to prevent fluid blow-by, maintain pedal pressure, and resist heat degradation.

• Hydro-Max / Vacuum Booster Interface: Designed to bolt directly onto Hydro-Max® or vacuum-assisted booster units found on commercial truck chassis.

1. Spongy, Soft, or Sinking Brake Pedal

• Symptom: The brake pedal feels mushy or slowly sinks all the way to the floorboard under continuous pressure at a stoplight.

• Cause: Internal fluid bypass where fluid leaks past the primary or secondary EPDM piston cups inside the bore rather than building pressure.

2. Visible Brake Fluid Leaks & Low Reservoir Level

• Symptom: Fluid wetness around the master cylinder body, low brake fluid level in the reservoir, or fluid leaking at the mounting flange between the master cylinder and the booster.

• Cause: Hardened, cracked rear shaft seals or damaged fluid reservoir grommets allowing fluid to escape.

3. Brake Dragging or Locked Wheels

• Symptom: Brake pads remain continuously engaged against the rotors after releasing the pedal, causing excessive heat, burning odor, or poor vehicle acceleration.

• Cause: Clogged compensating ports inside the master cylinder bore preventing hydraulic fluid from returning to the reservoir upon pedal release.

4. Brake Warning Light & Contaminated Fluid

• Symptom: The dashboard BRAKE warning light illuminates, or the brake fluid in the reservoir appears dark, cloudy, or filled with rubber particulate debris.

• Cause: Hydraulic pressure imbalance detected between the front and rear circuits, or internal rubber cup seal disintegration.