Spongy brake pedal, ABS?

RAJ SUNDAR

MEMBER
2004 ACURA MDX
3.5L
V6
AUTOMATIC
200,000 MILES

The brake reservoir became empty due to a brake fluid leakage from the right front caliper caused by the banjo bolt hose connection.

After fixing the leak, the Master Cylinder (MC) was bench-bled before installation, and all four wheels were thoroughly bled. However, even though the brake pedal feels firm when the engine is off, as soon as the engine starts, a hissing sound occurs, and the brake pedal goes all the way to the floor.

While test-driving the vehicle at a speed of 100km/h, we observed that the brake stops the vehicle instantly when applied. However, after the stop, the brake pedal goes all the way to the floor without any resistance. In other words, the brake works when the pedal is pressed halfway down, and then it travels freely to the bottom.

No further fluid leak found anywhere in the system.

Before the leak, the vehicle's brake system was functioning well.

Initially, when I started to bleed the brakes, there was no fluid coming out. However, after driving the vehicle around in a large backyard and applying the brakes several times to activate the ABS, there was a lot of air in the fluid when I attempted to bleed the brakes again.

Do you think the air is still trapped in the system? Can you tell me what might be wrong?

5 Replies

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Hi,

If the brake pedal is going to the floor, there is a good chance the master cylinder has failed as a result of the fluid leaking.

Over time, the plungers in the MC ride the same distance. As a result, dirt and corrosion can develop in the sections not used. If the pedal suddenly goes to the floor as a result of a (in this case) a leak at the hose/banjo fitting, it could have damaged the plungers.

Try this: With the engine off, pump up the brakes until you get a firm pedal. Then, hold pressure on the pedal and see if it slowly goes to the floor. If it does, replace the MC.

Also, take a look at the attached pic for bleed sequence. Make sure you do it in that order.

Let me know.

Joe

See pic below.

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Wednesday, April 10th, 2024 AT 7:00 PM

RAJ SUNDAR

MEMBER
23 POSTS

Thank you for your prompt response, Joe.

I followed the recommended sequence, but as the vehicle was an Australian-release Honda MDX, I had to start from the front right. I selected the US equivalent Acura MDX as the option was not available while feeding the vehicle details.

I conducted the test you suggested and found that the pedal was very firm at the first 1/4th travel (with the engine off) and it did not go all the way to the floor even with full force. I also tested the pedal's firmness before connecting the lines to the MC during installation, plugging the out ports and pumping the brakes. The pedal remained very firm even when the engine was on.

My question is whether it is safe to assume that there is no internal leakage in the MC after it has been successfully bench-bled (bled until no air bubbles were visible).

So far, didn't get the ABS error light.

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Wednesday, April 10th, 2024 AT 9:51 PM

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Hi,

If there was a leak, I suspect you would see it. If the brake fluid level isn't dropping, I would think it's okay. However, if the pedal is still going to the floor with the engine running, there must be air in the ABS system.

If it still is going to the floor, take a look through this link and let me know if it helps:

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Let me know.

Joe

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Thursday, April 11th, 2024 AT 7:31 PM

RAJ SUNDAR

MEMBER
23 POSTS

Hi Joe,

I used the Foxcom NT630 Plus scanner to automatically bleed the ABS system. After that, I bled all four wheels again. However, the issue still persists.

I am wondering if the brake booster is faulty. Normally, a damaged diaphragm would cause the brake pedal to become very hard, but in my case, it feels spongy. Can the diaphragm be damaged in a way that causes the pedal to go all the way to the floor?

If I disconnect the vacuum hose from the brake booster while the engine is running, the brake pedal becomes hard to push.

I am eager to receive your response.

Regards

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Friday, April 19th, 2024 AT 5:50 AM

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Hi,

When you remove the vacuum from the booster, the assist is gone, so the idea that the pedal gets stiff is normal.

I suspect the problem is the brake master cylinder itself. If I recall, it still is the original one. There is a good chance the plungers inside were damaged by passing over sections inside the MC that were unused. It isn't uncommon.

Let me know your thoughts.

Joe

See pics below.

Images (Click to make bigger)