1997-2001 Mitsubishi Montero Brake Master Cylinder Symptoms

By Core Brake Parts Tech Team — Circleville, OH warehouse

A failing master cylinder on a 1997-2001 Mitsubishi Montero usually shows up as a brake pedal that slowly sinks while you hold steady pressure at a stop, a pedal that stays soft after a proper bleed, a lit red brake warning lamp, or brake fluid weeping from the back of the cylinder onto the booster. Before you buy one, rule out the two things that fool people on these trucks: a leaking front brake line (a known recall on the 1997-1998 Montero) and, on the 2001 model, a tired hydraulic booster accumulator.

What the master cylinder does on a Montero

The master cylinder is the hydraulic pump at the driver side of the firewall. When you press the pedal, the booster multiplies your foot force and pushes a rod into the master cylinder. Inside, two pistons with rubber cups pressurize two separate circuits, so a leak in one circuit still leaves you some braking from the other. The plastic reservoir on top holds the fluid that refills the bore and makes up for pad wear.

That dual-circuit design is why master cylinder trouble rarely feels like total brake loss. It feels like a pedal that goes lower than it should, or one that creeps toward the floor. The cups wear against the bore over years of use, and moisture-laden fluid pits the bore, so the seals stop holding pressure. Fluid then leaks past the cups internally, from the pressure side back to the reservoir side, with no drip on the ground at all.

One detail matters on the 2001 Montero: NHTSA recall data describes it as having a hydraulic brake booster with a nitrogen-charged accumulator, not a simple vacuum booster. That changes how you diagnose a hard or delayed pedal, covered below. If you own a Montero Sport instead of the full-size Montero, the parts are different, so always confirm the exact model on the VIN or door label before ordering.

Symptoms of a bad master cylinder

Cardone, which remanufactures master cylinders, lists four common symptoms on its master cylinder tech sheet: a brake warning light, a spongy or sinking pedal, contaminated fluid, and leaks. Here is what each looks like on the truck.

• Sinking pedal at a stop. Hold firm, steady pressure at a red light. If the pedal slowly drops toward the floor with no visible leak anywhere, internal bypass in the master cylinder is the prime suspect.
• Soft pedal that bleeding does not fix. Air makes a pedal spongy, and bleeding cures air. If the pedal is still soft after a correct bleed, the cylinder may not be building full pressure.
• Red BRAKE warning lamp. The lamp can come on for low fluid or a pressure difference between circuits. Check the parking brake first, then the reservoir level.
• Fluid on the booster or firewall. A wet stripe running down the booster shell behind the master cylinder, or bubbled paint under it, points to a leaking rear seal.
• Dark or dirty fluid. Fluid that is black, cloudy or full of debris means the seals and bore are being attacked, and a flush is overdue either way.

Problems that mimic a failing master cylinder

A low or dropping pedal is not proof of a bad master cylinder. On these Monteros, check these first.

The front brake line recall is worth singling out. NHTSA's record for the 1997 and 1998 Montero says the front brake lines can develop pin holes from chafing against the edge of the pass-through hole in the inner fender, and that fluid can leak, causing deteriorated braking and a lit brake lamp. That is exactly the symptom set people blame on the master cylinder. If your reservoir keeps going low, trace the front lines through the inner fenders before anything else. A rusted or chafed hard line should be replaced, and you can find replacement brake lines and brake hoses in our catalog.

For the 2001 Montero, NHTSA's recall record says the accumulator on the hydraulic brake booster may have been improperly manufactured and can slowly leak its pressurized nitrogen. The result is an abnormal noise from the left side of the dash when the brakes are applied and possibly a delayed first brake assist after the vehicle has been parked. The dealer remedy was an improved accumulator. A hard pedal on the first stop of the day is an assist problem, not a master cylinder problem.

How to confirm the master cylinder is the problem

1. Check the fluid. Note the level against the reservoir marks and the color. A level that stays put while the pedal sinks suggests internal bypass. A level that keeps dropping means fluid is leaving the system somewhere.
2. Look for external leaks. Inspect the joint between the master cylinder and booster, every line fitting, the front lines at the inner fenders, the hoses and each wheel.
3. Do the steady-pressure test. Engine running, foot on the pedal with moderate, steady force and hold it. A slow, steady sink with no leak found is the classic internal-bypass sign.
4. Isolate the cylinder. A technician can cap the master cylinder outlet ports and press the pedal. If the pedal is firm with the ports capped, the cylinder is holding and the fault is downstream. If it still sinks, the cylinder is bad.
5. Check recall status. Run the VIN on nhtsa.gov/recalls. Open recalls are repaired by a dealer at no cost.

Replacing it the right way

Swapping a master cylinder is a bolt-on job, but the bleeding is where most DIY replacements go wrong. Cardone rates the replacement for a service technician or higher, at under 1.5 hours. Its instructions give the key steps:

• Transfer parts. Move any proportioning valves, switches or other components from the old cylinder to the new one.
• Bench bleed first. Support the cylinder level on the bench, never clamped by its body, fill the reservoir with new fluid from a sealed container, and slowly stroke the piston with a dowel in short strokes of 3/4 inch to 1 inch, never more than 1 inch, until it is firm with less than 1/8 inch of travel. On step-bore quick take-up designs, wait 20 seconds between strokes. Units with a remote reservoir are bled on the vehicle instead.
• Keep the plugs in. Leave the bleeder plugs in the ports until the cylinder is mounted, then remove one at a time as you connect each line.
• Bleed the whole system per the factory service procedure, and do not drive the truck until the pedal feel and travel are correct.

If the new cylinder felt firm with the plugs installed on the bench but the pedal goes soft once installed, Cardone notes the replacement cylinder is not the problem: look for air or a fault elsewhere. Wagner adds a common trap: pushing the pedal all the way to the floor during manual bleeding can let a piston stick in its forward position from a vacuum lock, giving a pedal that drops with little fluid at the bleeders. Bench bleeding first and following the factory bleed procedure, including any separate ABS bleed, prevents it.

What to replace at the same time

• Brake fluid. Cardone advises flushing the entire system with the manufacturer-approved fluid whenever brake components are replaced, and never mixing DOT 3 or DOT 4 (glycol based) with DOT 5 (silicone). Use the grade printed on your reservoir cap or in the owner's manual.
• Lines and hoses. On a truck this age, cracked rubber hoses and corroded hard lines are common. Inspect them while the system is open.
• The booster, only if it tests bad. A hard pedal or hissing points to the booster, not the master cylinder. Diagnose before you buy.

Shop brake master cylinders and brake boosters, and match the part to your VIN. If you drive the Montero Sport rather than the full-size Montero, we carry a vacuum power brake booster for the 1997-2001 Montero Sport.

Frequently Asked Questions

Can I drive a Montero with a bad master cylinder?

You should not. A cylinder that bypasses internally can get worse without warning, and a sinking pedal means longer stops. Park it and repair it, or tow it to a shop.

Why does my Montero brake pedal sink but there is no leak?

A slow sink under steady pressure with no external leak and a steady reservoir level is the classic sign of fluid bypassing the master cylinder piston cups internally.

Is the brake noise from my 2001 Montero dash the master cylinder?

Usually not. NHTSA recall 01V254 covers a hydraulic booster accumulator on the 2001 Montero that can lose nitrogen, causing noise from the left side of the dash and delayed assist. Check the recall by VIN.

Do I have to bench bleed a new master cylinder?

Yes, unless it has a remote reservoir. Cardone says all master cylinders need a bench bleed followed by a full system bleed after installation.

How often should Montero brake fluid be changed?

Follow the interval in your owner's or service manual. Cardone notes most manufacturers recommend replacing brake fluid every 2 years and suggests inspecting fluid and hoses every 10,000 to 15,000 miles.

Sources

1. NHTSA recall data, 1997 Mitsubishi Montero - recall 98V220, chafed front brake lines, fluid leak and brake lamp
2. NHTSA recall data, 2001 Mitsubishi Montero - recall 01V254, hydraulic booster accumulator, dash noise and delayed assist
3. Cardone master cylinder tech sheet - common symptoms, flush and fluid rules, skill level and time, fluid interval
4. Cardone bench bleeding procedure - bench bleed steps, stroke lengths, step-bore wait, soft pedal after install
5. Wagner Brake technical tip - stuck piston and no-pedal condition after master cylinder installation