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Old 07-19-2018, 04:08 PM   #1
hydromech
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2000 SL1
Default Low and spongy brake pedal

I recently replaced a rusted brake tube to rear wheel with Ni-Copp tube then bleeded that wheel as usual. The fluid level in the Master Cylinder never got too low. With new tube I'm not loosing fluid any more - so no more leaks.

Now the car stops but the brake pedal is kinda low. Also I can feel I can push it even lower after a complete stop.

What is this: still air in the system, Master Cylinder seals, brake Booster ?

How to isolate the problem ? How to test each component ?


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Old 07-19-2018, 04:24 PM   #2
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1999 SL2
Default Re: Low and spongy brake pedal

Sounds like air in the lines. Air will compress more than fluid and let you press the pedal down farther.

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Old 07-19-2018, 05:34 PM   #3
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Default Re: Low and spongy brake pedal

A proper brake system service after replacing any brake line is to bleed the other half of the brake line associated with the replaced line. Safety design incorporates a dual diagonal brake system - left front/right rear, right front/left rear, tandem pistons with two output ports in the master cylinder. Examine your brake master cylinder. It should have two brake lines, one for each half of the brake system. Just because you bled the line doesn't mean air may remain in the other half on that same line. Brake bleeding usually requires a full bled on at least half the system, either the rf and lr or the lf and rr. You bled only 25% of your system. While you're performing bleeding, it doesn't hurt to complete a full system bleed if brake fluid was never replaced. Flushing out old fluid should be performed every time front disc brakes are replaced. Dark brown brake fluid is old. Light honey colored fluid is new. Be prepared to learn and accept whether or not the master cylinder is worn out too after a full system bleed.

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Old 07-20-2018, 09:34 AM   #4
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Default Re: Low and spongy brake pedal

Thanks for reply

So I will bleed all 4 lines in recommended sequence.

But if after that I still have soft pedal - does that mean bad Master Cylinder ?

How to test Master Cylinder for sure ?

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Old 07-20-2018, 10:41 AM   #5
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2003 L-Series 3.0L Sedan
Default Re: Low and spongy brake pedal

Google brake master cylinder test. When a brake system is fully bled of air, pressing the brake pedal (engine off) should result in a firm pedal. Hydraulics relies on building instant oil pressure to generate forces acting on brake pistons that are driven against brake shoes and brake discs to stop a vehicle. If you need examples, try brakes on working cars.

Never have the engine running as vacuum is supplied to the vacuum brake boost unit - power assisted braking. You're testing the brake master cylinder and not the master cylinder and power assist unit that interferes with testing. With engine off, press and release the brake pedal several times as you should hear a slight hissing sound - the power brake unit vacuum reservoir is depleted on every brake pedal application until vacuum is depleted (engine not supplying vacuum). Each time pedal is pressed, power assist decreases as pedal resistance goes up until power assist is lost. Pedal travels less. After three to five pedal applications the pedal firms up in about half its travel length. At this point brakes are all hydraulics (zero power assist) and you should be able to stand on the brake pedal as if your life depended on it - without power assist your life depends on the brake master cylinder to generate hydraulic pressure to operate brakes. The pedal should never move towards the floor. Every car, truck, and suv has the same hydraulic brakes operating the same way. With engine running, power assisted braking comes from the vacuum powered brake boost unit amplifying leg power to generate additional force against the brake master cylinder rod as we press on the brake pedal. Never test brake master cylinders with engines running.

If the brake pedal goes to the floor, either brakes weren't bled correctly or the master cylinder is worn. This presumes there aren't any leaks in the brake lines.

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Old 07-20-2018, 11:43 AM   #6
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2003 L-Series 3.0L Sedan
2002 SL2
Default Re: Low and spongy brake pedal

If the pedal wasn't low before the pipe replacement, then that is clearly air in the system. Only you know how the pedal felt before. If it was low before the pipe replacement, then you have another issue, but it sounds like it's just needs bleeding.

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