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  #1  
Old 07-20-2013, 10:40 AM
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240D Brakes: How do I bleed the Master Cylinder?

I have just replaced the brake fluid and have bled the system through the screws at the four calipers (RR first, then RL, FR, FL). Unfortunately, there still seems to be air in the system: when braking, the pedal now goes all the way down the first time with no braking effect, the second time I can feel the brake pads engaging, and the third time the car slowly stops (with the pedal going all the way down).

Is the master cylinder on a 1979 240D bleedable at all? My FSM says (for the 240D):
"To remove all air bubbles from tandem main cylinder, step down fully on brake pedal three times while bleeding with bleed screws of brake pedal opened."

However, I cannot see any bleed screws on either the master cylinder or the brake pedal. Can someone chime in and help me out on this one?

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  #2  
Old 07-20-2013, 01:35 PM
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There should be a lot of sites that have "Bench Bleeding The Master Cylinder" or Bleeding The Master Cylinder.

For about $4-$8 you can buy a Kit in the Auto Parts Store. It has some Plastic Fittings and tubing. You unscrew the Brake Lines at the Master Cylinder and screw in the Plastic fittings and route the Tubing so that the Fluid goes back into the Master Cylinder. You fill it up with Brake Fluid and Pump till you don't see any Air Bubbles any More. People claim that when you do the Pumping on the Master Cylinder the Engine should be running.
When the above is done you install the Brake Lines to the Master Cylinder and proceed to bleed your Brakes.

This is what I do when the MC is on the Car but you need a Helper.
I put a bunch of Old Rags under the Brake Lines where the come out of the Master Cylinder.
You need to have a Master Cylinder Full of Brake Fluid.
Step one-Loosen the Brake Line Nuts at the Master Cylinder, start the Car and have someone step slowly on the Brake pedal and hold the Pedal down. Air of Brake Fluid will come out.
Step 2-Then tighten up the Brake Line fittings a bit and have the Helper release the Pedal.
Go back to Step one and repeat the steps always ending with Step 2 until you think all the Air is out of the Master Cylinder and tighten up the Brake Lines and proceed to bleeding the Brakes.

If you have done a good job on the above stuff and you Pedal is still sinking it is time to look for a New Master Cylinder.

Please note that the above stuff is done after the Car has sat and everything has cooled off. I am saying that incase you Brake really is dragging because you don't want overheated Brake Fluid to influence the outcome.

Things that cause Dragging Brakes. Caliper Piston sticking in the Bore; The Brake Hose messed up inside acting as a one way Valve; and, inside of the Master Cylinder there is fluid return holes. If the holes are plugged the Brake Lines will remain pressurized.
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  #3  
Old 07-20-2013, 03:55 PM
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If you plan on bleeding your brakes every other year as I believe is recommended I would invest in a power bleeder. I have had this one for several years ow and it makes bleeding the brakes a snap. I bleed the 124 in about 20 min or so. No muss, no fuss.
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  #4  
Old 07-20-2013, 05:03 PM
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Quote:
Originally Posted by davidmash View Post
If you plan on bleeding your brakes every other year as I believe is recommended I would invest in a power bleeder. I have had this one for several years ow and it makes bleeding the brakes a snap. I bleed the 124 in about 20 min or so. No muss, no fuss.
X2
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  #5  
Old 07-20-2013, 05:39 PM
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Quote:
Originally Posted by davidmash View Post
If you plan on bleeding your brakes every other year as I believe is recommended I would invest in a power bleeder. I have had this one for several years ow and it makes bleeding the brakes a snap. I bleed the 124 in about 20 min or so. No muss, no fuss.
+1

I have a swollen MC grommet that needs to be replaced on my 240D, and this is on my to do list...

I have the motive bleeder with the screw-on cap. Works perfect.
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  #6  
Old 07-20-2013, 08:16 PM
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always bench bleed new installs. then power bleeder really helps. change flex hoses if they haven't been done.
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  #7  
Old 07-20-2013, 08:25 PM
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I have had a Mighty Vac type hand held Vacuum Pump since the 1980s. Way back then I took a Jelly Jar and drilled 2 Holes in it and shoved some Plastic Vacuum fittings into the Holes with Epoxy around them.
I use that in conjunction with some Vacuum Hoe and use the Hand Held V Pump to bleed the Brakes. The old Fluid goes into the Jar.

But, lately what I do on the Mercedes is I build up the Vacuum and get the Brake Fluid in the Line moving and I disconnect the Hand Held V Pump and walk away for awhile. Gravity does the rest of the Bleeding job.
It is only needed to return and check the Fluid Level in the Master Cylinder.

The only thing I don't like about the setup is the Glass Jar that can break. But, since the 1980s I have managed not to break the Jar.
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  #8  
Old 07-20-2013, 11:51 PM
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I've had good results with Bench Bleeding a new or replacement Master Cylinder.

I look forward to trying out the power bleeders next time I need to perform the operation.
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  #9  
Old 07-21-2013, 07:09 AM
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I replaced my master cylinder a few months ago. I don't remember if I bench bled it before I put it on. I did vacuum bleed the entire system after I installed it, and results have been good.
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  #10  
Old 07-21-2013, 12:24 PM
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Quote:
Originally Posted by davidmash View Post
If you plan on bleeding your brakes every other year as I believe is recommended I would invest in a power bleeder. I have had this one for several years ow and it makes bleeding the brakes a snap. I bleed the 124 in about 20 min or so. No muss, no fuss.
OK, guys, you got me convinced. Can you suggest a good value power bleeder and best place to get it? Also, what's the procedure for bleeding master cylinder + brake lines with the power bleeder? Thanks..
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  #11  
Old 07-21-2013, 12:29 PM
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Sounds like you let all the fluid out versus bleeding until the system was flushed. Dont do the former unless you want a headache!
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  #12  
Old 07-22-2013, 10:13 AM
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Quote:
Originally Posted by winmutt View Post
Sounds like you let all the fluid out versus bleeding until the system was flushed. Dont do the former unless you want a headache!
What happened is that I had forgotten about the little reservoir (rear calipers) and it got emptied while I was bleeding the rear calipers (I was only checking the level of fluid in the main reservoir). I had done this procedure several times correctly in the past, but for some reason I forgot and made this mistake this time around. Then realizing my mistake, I proceeded to fill that reservoir up and did the bleeding procedure for both rear and front calipers, but obviously air had already entered the system at that time.
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  #13  
Old 07-22-2013, 10:14 AM
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So any suggestions for a good value bleeder and where to get a good deal on it?
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  #14  
Old 07-22-2013, 11:47 AM
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Somewhere in the DIY Section someone made a Power Bleeder from a Garden/Weed Sprayer.
DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum
PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles
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  #15  
Old 07-22-2013, 04:09 PM
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85 300D 4spd+tow+h4
 
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Quote:
Originally Posted by rino View Post
What happened is that I had forgotten about the little reservoir (rear calipers) and it got emptied while I was bleeding the rear calipers (I was only checking the level of fluid in the main reservoir). I had done this procedure several times correctly in the past, but for some reason I forgot and made this mistake this time around. Then realizing my mistake, I proceeded to fill that reservoir up and did the bleeding procedure for both rear and front calipers, but obviously air had already entered the system at that time.
BTDT its a pita...

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1994 E320 Cabriolet
1995 E320 Touring
2001 Eurovan FOR SALE
1985 300D car, sold and missed.
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