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  #1  
Old 09-14-2018, 10:53 PM
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Brake goes to floor after MC replacement

So I changed the master cylinder because the brakes go to the floor with little effort. Install the master cylinder after I bench bleed and after install I bleed with a pressure bleeder. Brake ordeal still goes to the floor. The only odd thing was when I let pressure out of the pressure bleeder a couple bubbles came from the front of the master cylinder into the brake res. The hoses look good but could it be one has a leak? Or is the new master cylinder defective?


1981 Mercedes 300SD
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  #2  
Old 09-15-2018, 07:47 AM
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Did you bleed at all of the wheels? If not, do that.

Did the old MC suddenly go to floor or was this after some other brake repairs?
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  #3  
Old 09-15-2018, 09:52 AM
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Post Brake Bleeding

It sounds like you didn't get quite all the air out when you pressure bled the calipers .

I hope you know to use a bottle and hose, bleed all four calipers beginning with the right rear, then the left rear then the right front and finish with the left front .

You must bleed each caliper until not only are there no bubbles coming out the end of the hose but clear fresh fluid is coming out too .

Many only want to use one little shortie bottle of cheap brake fluid and that's often not enough and you must never pour the bled out fluid back into the fluid reservoir .
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Old 09-15-2018, 11:01 AM
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So I used three new bottles of fluid, I bled every wheel 3 times. The old mc held pressure but would slowly sink after stopping. I’m thinking I need to return the new mc and get another one. I couldn’t even bench bleed the first MC they sold me, no pressure in the front port.
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  #5  
Old 09-15-2018, 04:06 PM
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I would find another supplier.
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  #6  
Old 09-15-2018, 07:53 PM
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Once again from my post 2

Did you bleed at all of the wheels? If not, do that.

Did the old MC suddenly go to floor or was this after some other brake repairs?
~~~~
Also, when bench bleeding, what are you using to depress the piston and how much travel are you getting?
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  #7  
Old 09-15-2018, 09:10 PM
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There's a sequence for bleeding, rear opposite opposed first then next furthest away (other rear caliper).

If it's an oldie oldie with ABS this needs sorting too.
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  #8  
Old 09-16-2018, 02:22 PM
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Yes, I bled all 4 wheels 3 times each, yes I do bleed my brakes Pr,dr,of,dr. 198q so no abs brakes. I think my issues was buying rebuilt and not new. When I bought the car the brake pedal would sink and it showed leakage at the master cylinder rear onto the vac booster. When bleeding in the bench I use a wooden dowel sanded down on the end to the same size as the rod. I used a caliper to measure.
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  #9  
Old 09-16-2018, 02:46 PM
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Use a # 2 Phillips ( common size ) screwdriver to push the piston and see if you get same amount of travel as the wood rod. Push until you feel a hard stop.

There is a high likelihood you are not pushing the rear piston far enough for it to bottom out on the front piston allowing the front piston to bottom out.
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  #10  
Old 09-16-2018, 05:26 PM
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I fully bottomed it out.
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  #11  
Old 09-16-2018, 05:27 PM
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Brand new MC gets here Tuesday I will report back how it goes
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