02/07/2019
A soft or spongy brake pedal in your Mitsubishi Eclipse is more than just an annoyance; it's a significant safety concern that demands immediate attention. When you press the pedal and it feels like it's sinking too far, or provides little resistance, it indicates a problem within your braking system. This issue can dramatically increase your stopping distance, putting you and other road users at risk. Understanding the underlying causes is the first step towards a proper diagnosis and repair, ensuring your Eclipse remains safe and reliable on British roads.
The braking system in any vehicle, including your Mitsubishi Eclipse, is a complex hydraulic network designed to convert pedal pressure into stopping force at the wheels. Any compromise in this system's integrity, whether it's fluid contamination, air ingress, or mechanical wear, can manifest as a soft pedal. While many issues can contribute to this sensation, one common culprit, especially in vehicles that have gone without proper routine maintenance, is the absorption of moisture into the brake fluid.
- The Menace of Moisture-Contaminated Brake Fluid
- Diagnosing the Problem: Beyond Fluid Contamination
- Other Common Causes of a Soft Brake Pedal
- Comprehensive Diagnostic Steps for Your Eclipse
- Prevention and Maintenance: Your Best Defence
- Comparative Table: Soft Pedal Symptoms vs. Potential Causes
- Frequently Asked Questions (FAQs)
The Menace of Moisture-Contaminated Brake Fluid
Brake fluid is hygroscopic, meaning it naturally absorbs moisture from the atmosphere over time. This absorption is a gradual process, but its effects can be profound. As moisture accumulates, it significantly lowers the fluid's boiling point. Under the intense heat generated during heavy braking, this water can turn into vapour (gas). Unlike incompressible brake fluid, gas is compressible. When you press the brake pedal, instead of directly transferring pressure to the calipers and wheel cylinders, some of that pressure is wasted compressing the vapour bubbles. This results in that characteristic soft, spongy, or even sinking pedal feel, often referred to as 'brake fade'.
The key to preventing this issue lies in routine maintenance. Brake fluid flushes are often overlooked, but they are crucial. Manufacturers typically recommend flushing and replacing brake fluid every two to three years, or as specified in your Eclipse's owner's manual. Neglecting this simple service allows moisture levels to build up, degrading the fluid's performance and potentially leading to internal corrosion within the brake lines and components over time. This corrosion can further compromise the system's efficiency and lifespan.
Diagnosing the Problem: Beyond Fluid Contamination
While moisture in the brake fluid is a common cause, a soft pedal can stem from various other issues within the hydraulic braking system. A thorough diagnostic process is essential to pinpoint the exact problem. One crucial diagnostic step, especially when other common causes have been ruled out, is to check the booster pin gap.
Understanding the Brake Booster and Pin Gap
The brake booster, often called a servo in the UK, is a vacuum-assisted device designed to multiply the force you apply to the brake pedal, making it easier to stop the vehicle. It sits between the brake pedal and the master cylinder. Inside the booster, a pushrod (or pin) connects the pedal to the master cylinder's piston.
The 'booster pin gap' refers to the small clearance or adjustment between this pushrod and the master cylinder piston. If this gap is incorrect – either too large or too small – it can lead to problems. An excessively large gap means the pedal has to travel further before it begins to actuate the master cylinder, resulting in a sensation of excessive pedal travel and a soft, vague feel. Conversely, too small a gap can cause the brakes to drag or even be partially applied when the pedal is at rest.
Checking and adjusting this gap is a more advanced diagnostic step, typically performed by a qualified mechanic. It involves ensuring the pushrod effectively engages the master cylinder without preloading it or having excessive free play. Incorrect adjustment here can mimic other brake issues, making it a critical, though often overlooked, item in the diagnostic process for a soft pedal.
Other Common Causes of a Soft Brake Pedal
Beyond fluid degradation and booster pin gap issues, several other culprits can lead to a soft brake pedal in your Mitsubishi Eclipse:
1. Air in the Brake Lines
This is arguably the most common cause of a soft or spongy brake pedal. Air can enter the system through various means: opening the bleed nipples during brake pad/caliper replacement without proper re-bleeding, a low fluid level in the master cylinder reservoir allowing air to be drawn in, or even a slow leak in a brake line or component. Like water vapour, air is compressible, leading to the same spongy pedal feel. The solution is a thorough bleeding of the entire brake system, starting from the wheel furthest from the master cylinder and working inwards.
2. Worn or Failing Master Cylinder
The master cylinder is the heart of your hydraulic braking system. It converts the mechanical force from the brake pedal into hydraulic pressure. If the internal seals within the master cylinder wear out, fluid can bypass the piston, leading to a loss of pressure. This often manifests as a pedal that slowly sinks to the floor when constant pressure is applied, even if there are no external leaks. A failing master cylinder typically requires replacement.
3. Leaking Brake Caliper or Wheel Cylinder
Each wheel has a caliper (for disc brakes) or a wheel cylinder (for drum brakes) that applies pressure to the pads or shoes. If the seals in these components fail, brake fluid can leak out, leading to a drop in hydraulic pressure and a soft pedal. External leaks are usually visible as fluid stains on or around the wheels. A visual inspection for leaks is always a good starting point when diagnosing brake issues.
4. Collapsed or Swollen Brake Hoses
Over time, rubber brake hoses can deteriorate. The internal lining can collapse, restricting fluid flow, or the hose itself can swell under pressure. While a collapsed hose might cause a hard pedal, a swollen hose can sometimes lead to an inconsistent or soft feel, especially if it's not expanding uniformly. These hoses should be inspected for cracks, bulges, or signs of wear during routine service.
5. Vacuum Leak in the Brake Booster System
The brake booster relies on engine vacuum to assist braking. A leak in the vacuum hose connecting the booster to the engine, or a faulty diaphragm within the booster itself, can reduce or eliminate the power assist. While a complete booster failure often results in an extremely hard pedal, a partial vacuum leak might lead to an inconsistent or slightly soft pedal feel, as the assistance isn't fully effective.
Comprehensive Diagnostic Steps for Your Eclipse
When faced with a soft brake pedal, follow a systematic approach to diagnosis:
- Check Brake Fluid Level: Begin by checking the master cylinder reservoir. A low fluid level indicates a leak somewhere in the system. Top up with the correct DOT-specified fluid (e.g., DOT3 or DOT4, as recommended for your Eclipse).
- Inspect for Leaks: Visually inspect all brake lines, hoses, calipers, and wheel cylinders for any signs of fluid leaks. Check around the master cylinder as well.
- Brake Fluid Condition: Observe the colour of the brake fluid. Dark, murky fluid suggests contamination and warrants a flush. Clear fluid doesn't rule out moisture, but dark fluid is a definite sign of degradation.
- Bleed the Brakes: If no external leaks are found and the fluid looks okay, the next step is often to bleed the brakes. This removes any air that might have entered the system.
- Test Brake Booster: With the engine off, pump the brake pedal several times until it becomes firm. Then, hold the pedal down and start the engine. The pedal should sink slightly. If it doesn't, or if it feels unusually hard, there might be a booster issue or a vacuum leak.
- Check Booster Pin Gap: As mentioned, this is a more advanced check, typically performed if other issues are ruled out and a soft pedal persists. It ensures proper engagement between the pedal and master cylinder.
- Evaluate Master Cylinder: If the pedal slowly sinks to the floor while holding steady pressure, even after bleeding, the master cylinder is a strong suspect.
- Inspect Brake Hoses: Look for swelling, cracking, or damage on all flexible brake hoses.
Prevention and Maintenance: Your Best Defence
Preventing a soft brake pedal is far better than reacting to one. Regular maintenance is paramount for the safety and longevity of your Mitsubishi Eclipse's braking system:
- Routine Brake Fluid Flushes: Adhere to the manufacturer's recommended intervals for brake fluid replacement. This is typically every 2-3 years, regardless of mileage, to mitigate moisture absorption.
- Regular Inspections: Include brake system checks in your routine service schedule. This means inspecting brake pads, discs, calipers, lines, and hoses for wear, leaks, and damage.
- Use the Correct Fluid: Always use the type of brake fluid specified for your Mitsubishi Eclipse (e.g., DOT3 or DOT4). Mixing fluid types or using an incorrect type can cause damage to seals and compromise braking performance.
- Address Issues Promptly: Any change in pedal feel, noise, or braking performance should be investigated immediately. Do not postpone brake repairs.
By staying vigilant with your Eclipse's maintenance, you can significantly reduce the likelihood of encountering a soft brake pedal and ensure your vehicle stops reliably every time.
Comparative Table: Soft Pedal Symptoms vs. Potential Causes
| Symptom Description | Most Likely Cause(s) | Diagnostic Tip |
|---|---|---|
| Pedal sinks slowly to floor under constant pressure, no visible leaks | Failing Master Cylinder (internal leak) | Hold pedal down firmly for 30 seconds; if it sinks, suspect master cylinder. |
| Spongy, vague pedal feel; multiple pumps needed for firm pedal | Air in Brake Lines | Bleed brakes thoroughly; check for low fluid level. |
| Pedal feels soft, especially after heavy braking or on hot days | Moisture-Contaminated Brake Fluid | Check fluid colour (dark indicates contamination); consider a fluid flush. |
| Brake fluid level drops, visible fluid around wheels or lines | Leaking Caliper/Wheel Cylinder or Brake Line/Hose | Inspect all brake components for external leaks. |
| Inconsistent pedal feel, sometimes soft, sometimes okay | Incorrect Booster Pin Gap / Partial Vacuum Leak | Professional diagnosis required for booster pin gap; check vacuum lines to booster. |
| Noticeable loss of power assist, pedal hard to push but also feels 'empty' | Faulty Brake Booster (vacuum leak or internal diaphragm failure) | Perform engine-off/engine-on booster test. |
Frequently Asked Questions (FAQs)
Q1: Can I drive my Mitsubishi Eclipse with a soft brake pedal?
A: No, it is highly inadvisable and dangerous to drive with a soft brake pedal. Your stopping distance will be significantly increased, and you could lose braking ability entirely. You should have the issue diagnosed and repaired immediately by a qualified mechanic.
Q2: How often should I flush the brake fluid in my Eclipse?
A: While specific recommendations can vary, a general guideline is every 2-3 years or every 30,000-50,000 miles, whichever comes first. Always refer to your Mitsubishi Eclipse's owner's manual for precise recommendations.
Q3: What type of brake fluid does my Mitsubishi Eclipse use?
A: Most Mitsubishi Eclipse models typically use DOT3 or DOT4 brake fluid. It is crucial to check your owner's manual or the cap on your master cylinder reservoir to confirm the correct type, as using the wrong fluid can damage your braking system.
Q4: Can a soft brake pedal be caused by worn brake pads?
A: Worn brake pads typically cause a different pedal feel, often a longer pedal travel before engagement, sometimes with grinding noises or vibrations. While extremely worn pads can contribute to a slightly lower pedal, a genuinely 'soft' or 'spongy' pedal is almost always due to issues within the hydraulic system (air, fluid, master cylinder, leaks), not just pad wear.
Q5: Is bleeding the brakes a DIY job for a soft pedal?
A: Bleeding brakes can be a DIY job for experienced enthusiasts, but it requires proper tools and knowledge to do correctly. If done improperly, it can introduce more air into the system, worsening the problem. Given the critical safety aspect of brakes, if you're unsure, it's always best to have a professional perform the procedure.
In conclusion, a soft brake pedal on your Mitsubishi Eclipse is a clear warning sign that should not be ignored. Whether it's the common issue of moisture-laden brake fluid, air in the lines, a failing master cylinder, or a subtle booster pin gap problem, prompt and accurate diagnosis is key. Regular maintenance, particularly brake fluid flushes, is your best defence against these issues, ensuring your Eclipse provides safe and reliable stopping power for many miles to come.
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