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  #1  
Old 08-14-2016, 02:06 AM
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Question Bleeding master cylinder after installing?

My brake feels spongy. I bled all four brake lines again and it seems to be the same. Is it possible that I did not bleed the master cylinder properly? If I did not, how to fix it?

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  #2  
Old 08-14-2016, 07:21 AM
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If you bled all the wheels correctly the master should be fine too. How did you bleed them?
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #3  
Old 08-14-2016, 07:40 AM
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You can do the MC on the car by opening the lines at the MC. However, if the MC has an extreme upwards tilt, there might still be a bit of trapped air.

You can also do a reverse "bleed" by extending the caliper pistons and pushing them to the bottom of their bores.

Did you replace the brake pads at the same time? If so, let them wear in a bit because a slightly tilted pad will spring back and forth giving a soft pedal.
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Old 08-14-2016, 07:49 AM
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I've never had to bench bleed a Mercedes MC. But I've occasionally run into problems because there are front/rear chambers molded into to reservoir...one can run dry, but not the other. So now when I bleed the system, I keep the reservoir full to the brim. When I'm done bleeding, I use my Mityvac to remove enough fluid to get it to the fill line. Others may suggest a power bleeder. Also, be sure you start with the right rear, then left rear, right front, left front.
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Old 08-14-2016, 08:32 AM
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Replaced two master cylinders, normal cause foot on pedal which slowly descends.

I used a pressure type kit, attached tyre at 22psi opening the line furthest away first, repeat as necessary. Messy, as the caps never fit properly but eventually works.

If your vehicle has ABS there's an extra step as that can trap air.
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  #6  
Old 08-14-2016, 10:54 AM
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How did you bleed the master cylinder? I find it takes forever to bleed the front cylinder (rear brakes) pumping by hand or pedal.

Sixto
83 300SD
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Old 08-14-2016, 11:09 AM
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Does the instruction manual on YOUR new MC say ' bench bleed ' ?
This has been discussed a lot in the archives...
use ' bench Napa ' for the search words...
When I tried to skip the bench... I had to take the MC back off and do it with the NAPA kit held in a vise...usually if they say ' bench bleed' it will save you time to believe them.
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  #8  
Old 08-14-2016, 11:35 AM
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Quote:
Originally Posted by sixto View Post
How did you bleed the master cylinder? I find it takes forever to bleed the front cylinder (rear brakes) pumping by hand or pedal.

Sixto
83 300SD
Sixto, this is what I have used over the years, works well with no problems so far other than leaky caps.

My recommendation would be to modify it out of the box (new cap fitting) or opt for a more expensive version, this type is for home use really.

https://www.youtube.com/watch?v=Lz8t6ZkVSQM
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  #9  
Old 08-14-2016, 12:30 PM
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I have over the years rebuilt clutch and brake master cylinders and twice have had trouble bleeding systems.
My fault was the seals and bores were not lubed enough with fluid to let the internal spring push the piston completely back on its stroke.
So for instance on a clutch system the piston moved forward as the pedal was pushed and then the pedal just flapped back and forth , repeated taps with a hammer eventually see it move.
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Old 08-14-2016, 01:30 PM
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If the Master Cylinder is the old one some say to start the Engine so that when you sep on the pedal the pistons in the Master Cylinder do not travel into territory inside over the gunk or pits that they don't go over in normal use.

Don't know if it is still called bench bleeding when it is still on the Car. But, pile some rages under the Master Cylinder to catch the Brake Fluid that is going to leak out.

Loosen all of the fittings at the Master Cylinder (Engine running) enough so that fluid can come out of them. Have someone push down on the Brake Pedal all the way and hold it there Fluid should come out and then stop when the Brake Pedal bottoms out.
Then you tighten the fittings at the master cylinder then the brake pedal is released and it comes all they way back up sucking fluid down into the Master Cylinder.

Again loosen the fittings at the Master Cylinder Have someone press all the way down on the Brake Pedal and hold it there. Fluid should come out and then stop when the Brake Pedal bottoms out. Then you tighten the fittings at the master cylinder then the brake pedal is released and it comes all they way back up sucking fluid down into the Master Cylinder.

Repeat the above until you feel you are only getting a solid stream of fluid as the brake pedal is press down. Then you tighten the fittings at the master cylinder then the brake pedal is released and it comes all they way back up sucking fluid down into the Master Cylinder. You are done.

You can do the same as the above on each caliper starting with the most distant Caliper from the Master Cylinder working you way through the other Calipers and ending with the Caliper closest to the Master Cylinder.
But you will be loosening and tightening the Caliper Bleed Screw. Loosen Bleed Screw before stepping pushing down brake pedal, push down on brake pedal and hold it while the bleed screw is tightened, release brake pedal and repeat till you get a steady stream of fluid out of the bleed screw and tighten.

When I was a Kid I had to do the pedal pushing for my Dad. Boring and tedious but it works.
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Old 08-14-2016, 03:38 PM
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I don't know what is causing the sponginess. The car will stop fine but it is not as firm as I would like. I redid the bleeding from Right rear, left rear, right front, and left front and it is the same feel. So I suspect is it the master cylinder. Is there any way to fix it by keeping bleeding the brake lines or I have to remove the master cylinder and do a bench bleed again?
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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
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  #12  
Old 08-14-2016, 05:21 PM
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Quote:
Originally Posted by ah-kay View Post
or I have to remove the master cylinder and do a bench bleed again?
AGAIN ???... you bench bled it before installing it ? ...

Properly ???

Tell us what you did on the bench...

did you have the fittings to go into the ' out ' holes of the MC and those have 'hoses' on them which you directed back into the main chamber ? and move the cylinder vigorously until there were not air bubbles ?
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Old 08-14-2016, 08:29 PM
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Quote:
Originally Posted by ah-kay View Post
I don't know what is causing the sponginess. The car will stop fine but it is not as firm as I would like. I redid the bleeding from Right rear, left rear, right front, and left front and it is the same feel. So I suspect is it the master cylinder. Is there any way to fix it by keeping bleeding the brake lines or I have to remove the master cylinder and do a bench bleed again?
Do it as someone explained above.

Use pressure bleeder and first just crack the connections on the M/C and bleed one at a time until no bubbles come out. You want to do this here rather than try and push those air bubbles all the way to the calipers.

Once you see no bubbles at M/C, then pressure flush/bleed to each caliper. Make sure the tube off the caliper is dipping into fluid in a clear pop bottle so you can see the bubbles.

I would buy 4L (a gallon) of fluid and flush 500cc or more from each caliper. This so you get the air from the M/C that is probably in your lines all the way to the bleeders.

No need to remove M/C!
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  #14  
Old 08-14-2016, 08:46 PM
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I found that gently tapping the MC with a rubber hammer helped shake the bubbles loose that were stuck inside it. I ended up using a pressure bleeder, but instead of filling thr bleeder, just filling the reservoir and pressurizing that. It took a bit longer since after each corner I had to stop and fill the reservoir up. Just dont let the reservoir run out.
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  #15  
Old 08-15-2016, 03:32 PM
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Why remove it? I never "bench-bleed" on the bench since the pedal is much easier. All you need to do now is unbolt the 3 tubes and connect 3 stub tubes that route back to the reservoir. You can buy these in plastic parts cheap at auto parts. Clear plastic tubing is best to watch for bubbles. Keep pumping the pedal all the way down until you see no bubbles circulating thru the tubes. Might take 50 pumps, watch youtube. Then just reconnect the 3 factory tubes. You don't need to bleed those again. You should see fluid still at the top of each. If not, fill to the top (syringe, etc).

The diagnostic for air in the system is that you see a geyser of liquid shoot up into the reservoir as you release the pedal. That is easily visible in older cars w/ an open reservoir, but don't know if you could see it in yours. Thoroughly hose off any spilt brake fluid since it is a great paint remover, the exception is those smart enough to use silicone fluid (me).

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