W140 S420 Brake Bleeding: The Unique 1995 Method

Maintaining the braking system of any vehicle is paramount for safety, and for owners of the iconic Mercedes-Benz W140 S420, this task can present its own unique set of challenges. While generally known for their robust engineering, the 1995 model year W140 S420, in particular, has a peculiar characteristic in its anti-lock braking system (ABS) and acceleration skid control (ASR) pump, which can make a seemingly straightforward task like brake bleeding surprisingly complex. This guide aims to demystify the process, providing a tested and successful method for brake bleeding, specifically tailored for this distinctive model year.

Understanding Your W140 S420's Brake Pads

Before delving into the intricacies of bleeding, let's address a fundamental question: where are the brake pads located on your 1995 W140 S420? The brake pads are housed within the brake calipers, which are mounted over the brake discs (rotors) at each wheel. There are two brake pads per caliper, one on each side of the disc. When you press the brake pedal, hydraulic pressure forces the caliper pistons to squeeze these pads against the rotating discs, creating friction that slows or stops the vehicle. Accessing them typically involves removing the road wheel and then unbolting or unpinning the caliper assembly. While replacing brake pads is a separate task from bleeding, ensuring they are in good condition is always the first step in comprehensive brake maintenance.

The Peculiarities of the 1995 W140 S420 Braking System

The core of the brake bleeding dilemma for the 1995 W140 S420 lies in its specific ABS/ASR pump, identified by part number 0024319612. While this part was later superseded by 0024319812 (though verification of exact interchangeability isn't always straightforward), what is absolutely crucial to understand is that this particular model year is, to my knowledge, the only one that does not have an SP port. This seemingly minor detail has massive implications, as nearly every factory manual, including the EPC WIS, and even third-party resources like AlldataDIY, refer to this SP port as an integral part of their brake bleeding procedures for ASR-equipped models. Without it, standard, widely published methods simply won't work as intended, leading to frustrating and potentially dangerous results.

The Initial, Unsuccessful Bleeding Attempt

Like many, my first attempt at bleeding the brakes on my 1995 W140 S420 followed conventional wisdom and published procedures, adjusted for the missing SP port. This was undertaken after replacing the rear flexible brake lines and the front calipers, pads, and associated hardware. To prevent fluid leakage and air ingress when lines were uncapped, rubber plugs from a Miti-vac kit were used – a highly recommended practice.

The process began by topping off the brake fluid reservoir with Pentosin Super Dot 4. The male nipple on the master cylinder was opened. With the car running, a pressure bleeder attached to the fluid reservoir via a screw-on cap attachment was pressurised to 20 PSI, ensuring all air bubbles were purged before proceeding. The bleeding sequence then followed the traditional pattern: passenger rear, driver rear, front passenger, and finally, front driver.

At each wheel, an assistant maintained the pressure bleeder at 2 PSI while the bleeder valve was opened until no air bubbles were visible. Despite meticulous adherence to this procedure, the outcome was far from ideal. I was left with a critical problem: the front passenger and rear driver calipers were inexplicably stuck closed. Furthermore, with the car running and put into gear, attempting to turn the driver rear wheel caused the ABS pump to make a loud noise, akin to a hard brake application, and the dashboard illuminated with the ABS, ASR, and Brake warning lights. This indicated a fundamental failure in the bleeding process, clearly demonstrating that the standard approach was incompatible with this unique system.

The Breakthrough: A Working Bleeding Method

After extensive research and a frustrating period of trial and error, a successful method emerged. This procedure, while initially similar, incorporates a critical change that addresses the W140 S420's unique hydraulic system.

Preparation and Initial Steps (Identical to First Attempt):

1. Ensure your vehicle is safely raised on jack stands or a lift, with wheels removed for access to the calipers.
2. Top off the brake fluid reservoir with Pentosin Super Dot 4.
3. Attach a pressure bleeder to the fluid reservoir, ensuring a secure, air-tight seal.
4. Pressurise the reservoir to approximately 20 PSI. Allow this pressure to stabilise and ensure no air bubbles are coming from the reservoir.

The Critical Change – Manual Pumping with Engine Running:

This is where the procedure significantly diverges and becomes effective. While initially attempting to use a second vacuum Miti-vac at the wheel simultaneously with the pressure bleeder, it became clear that very little fluid was being expelled. This pointed to an issue with fluid movement or valve actuation within the ABS/ASR unit.

Important Note: This specific method involves aggressive brake pedal pumping. It is generally not recommended unless you have a new or close to new master cylinder. An older, worn master cylinder could be damaged by this procedure.

5. Remove the pressure bleeder from the fluid reservoir and ensure the reservoir is completely filled to the MAX line. Re-install the OEM reservoir cap securely.
6. Start the engine. It is crucial that the engine is running throughout this entire process. This ensures the ABS/ASR pump and other system components are active and functioning.
7. Enlist an assistant to operate the brake pedal.
8. Beginning with the passenger rear wheel (the wheel furthest from the master cylinder), prepare the bleeder valve with a 9mm brake line wrench.
9. Instruct your assistant to slowly and firmly pump the brake pedal nine times, as they would when coming to a stop.
10. On the tenth pump, have your assistant press the brake pedal all the way to the floor and hold it there firmly.
11. While your assistant holds the pedal to the floor, quickly open the bleeder valve using the 9mm wrench. As soon as a clear, bubble-free stream of fluid emerges, immediately close the valve. This is a very quick open-close action, taking only a fraction of a second.
12. Instruct your assistant to release the brake pedal once the valve is closed.
13. Repeat steps 9 through 11 two more times for the passenger rear wheel. This ensures a thorough flush and removal of any trapped air.
14. Before moving to the next wheel, remember to continuously monitor the brake fluid reservoir level. It is absolutely vital that the fluid level does not go low during this process, as this would introduce air back into the system. Top up as necessary.
15. Proceed to the remaining wheels in the standard sequence: driver rear, then front passenger, and finally, front driver. The front passenger wheel, in my experience, often required the most attempts to achieve a bubble-free stream, indicating it might be a common point for trapped air.

Upon completion of this procedure at all four wheels, the difference in brake feel was immediate and profound. The brakes felt better than they ever had, and crucially, none of the previously illuminated dashboard warning lamps (ABS, ASR, Brake) reappeared. This method successfully bypassed the need for an SP port and effectively bled the unique 1995 W140 S420 ABS/ASR system.

Why This Method Works

The success of this manual pumping method, particularly with the engine running, likely stems from its ability to properly actuate the internal valves and components of the specific 0024319612 ABS/ASR pump unit. Traditional pressure bleeding alone might not generate sufficient pressure differentials or cycle the internal solenoids in a way that effectively purges air from all passages, especially those within the ABS modulator. By pumping the pedal repeatedly and then holding it to the floor while quickly opening the bleeder, you are creating a forceful, controlled flow that pushes air out, potentially engaging the ABS pump's internal mechanisms in a way that mimics a hard braking event, which is known to purge air in some systems. The 'engine running' state ensures the pump is powered and ready to respond to these hydraulic demands.

Comparative Analysis: Failed vs. Successful Bleeding

To highlight the key differences and why the revised method is superior for the 1995 W140 S420, consider the following comparison:

Frequently Asked Questions (FAQs)

Q1: Do I need a Star Diagnostic Tool or similar device to actuate the ABS/ASR pump?

Based on the successful method described, no, you do not need a Star Diagnostic Tool or similar device to "actuate y61 switch other valve" for the 1995 W140 S420 with the 0024319612 ABS/ASR pump. The manual pumping method effectively bypasses the need for such electronic actuation.

Q2: What type of brake fluid should I use?

As per the successful procedure, Pentosin Super Dot 4 brake fluid was used and is highly recommended. Always check your vehicle's owner's manual for specific fluid requirements to ensure compatibility and optimal performance.

Q3: Why did my first bleeding attempt result in stuck calipers and dashboard lights?

The first attempt, relying on a standard pressure bleeding method, likely failed to adequately purge air from critical components within the unique 1995 W140 S420's ABS/ASR unit. Trapped air can prevent proper piston retraction or valve operation, leading to calipers remaining engaged (stuck closed) and triggering system fault lights.

Q4: Is this method safe for my master cylinder?

This method involves forceful pedal pumping. As noted, it is generally not recommended unless your master cylinder is new or in near-new condition. An older, worn master cylinder could potentially be damaged by the full stroke pedal depression involved in this procedure. If you have concerns about your master cylinder's condition, professional assessment is advised.

Q5: How often should I bleed my brakes?

Brake fluid is hygroscopic, meaning it absorbs moisture over time, which can degrade its performance and lead to corrosion. Mercedes-Benz generally recommends brake fluid flushes every two years, regardless of mileage. However, any time brake components are replaced or the system is opened, bleeding is essential.

Q6: Can I use this method for other W140 models or years?

This specific method has been proven effective for the 1995 W140 S420 with the 0024319612 ABS/ASR pump due to its unique lack of an SP port. Other W140 models or years might have different ABS/ASR systems that do include an SP port or require a diagnostic tool for proper bleeding. Always consult your vehicle's specific service manual for the correct procedure for your exact model and year.

Conclusion

Bleeding the brakes on a 1995 Mercedes-Benz W140 S420 can be a source of frustration for many due to its specific ABS/ASR unit and the absence of a crucial SP port. However, by understanding these unique characteristics and applying the proven manual pumping method with the engine running, you can achieve a perfectly bled braking system without the need for specialised diagnostic tools. This hands-on approach, while demanding careful attention to detail and fluid levels, ultimately delivers superior brake feel and restores confidence in your W140's stopping power. Always prioritise safety and ensure all steps are followed meticulously for optimal results.

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