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1991 300E24
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Yesterday my brakes just about stopped working! I only get very small amount of braking when my foot is planted on the floor, 4 days before I replaced the rear pads and bled the brakes thoroughly. Everything was working fine, I thought it may be air or something so I re bled them this afternoon. Still no change, on all brakes the fluid comes poring out when i pump the brakes, but pressure doesn't build up. Ask me whatever else you need to know, not exactly sure what you need to know. Friend mentioned something about the master cylinder?

Thanks
 

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2002 E55 AMG
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How did you bleed the system? If all you did was replace the pads, then Id think you either left a valve open or are bleeding it incorrectly causing air to get in.
 

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1991 420SEL, 1989 Porsche 928-S4, 1973 Lincoln Continental Town Car, 1960 Lincoln Continental Mark V
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If it's the master cylinder, normally you could see leaks. I'm not sure what the two small knobs on the top do, or why a master cylinder has an electrical connection! connection? Then again, it's a Mercedes! :D Maybe a fluid level sensor? This is really just a reservoir; unless it or it's attachments are leaking, it should be OK.

The booster is behind the cylinder, which takes the relatively small pressure you apply to the pedal and amplifies it to force the fluid into the calipers when applying the brakes (sorry if that's too basic). It's unusual, but they do fail sometimes. Those are an absolute bitch to work on, and I have no idea how to test it without removing.

You could check the pressure of the fluid coming out of the lines at each wheel. That wouldn't be definitive, but if the pressure is low (I don't know what "good" pressure is), then at least you know the problem is farther upstream.

The calipers have fittings, too, that can leak - but again; usually you can see the fluid.

Fluid loss, unless there is a gusher, is usually a gradual leak, though, not sudden.

Try topping off the fluid, park the car on clean dirt, driveway, or street, then stomp on the brakes as hard as you can. Hold it for several minutes. Then, check under the car for puddles and double check the fluid level to see if that's dropped.

You could try a "sneaky" diagnosis :eek:. Take it to a shop to get an estimate. If it sounds like something you could investigate or do yourself, blush and say you'll have to think about it. Then, go home and do it yourself. Even if you have to pay for the diagnosis, it would save crawling under, over, and through the dirt over several days/weeks yourself.
 

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Two '87 570SECs, one '87 560SEL
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Do the brakes work but it takes immense pedal pressure to stop the car? 1/2" black vacuum booster line from the engine to the brake booster is bad. This failure is invariably the white check valve in the middle of the 2' long line.

Does the brake pedal go all the way to to the floor without creating any stopping power? Master Cylinder or wheel cylinder bad. A bad wheel cylinder generally leads brake fluid all over the place. Could be one of the rubber lines from the chassis to the wheel that has decided to die and it expands when you push the pedal, collapsing back close to normal when the pedal is released.

Whaddya got??
 

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Do the brakes work but it takes immense pedal pressure to stop the car? 1/2" black vacuum booster line from the engine to the brake booster is bad. This failure is invariably the white check valve in the middle of the 2' long line.

Does the brake pedal go all the way to to the floor without creating any stopping power? Master Cylinder or wheel cylinder bad. A bad wheel cylinder generally leads brake fluid all over the place. Could be one of the rubber lines from the chassis to the wheel that has decided to die and it expands when you push the pedal, collapsing back close to normal when the pedal is released.

Whaddya got??
 

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Yesterday my brakes just about stopped working! I only get very small amount of braking when my foot is planted on the floor, 4 days before I replaced the rear pads and bled the brakes thoroughly. Everything was working fine, I thought it may be air or something so I re bled them this afternoon. Still no change, on all brakes the fluid comes poring out when i pump the brakes, but pressure doesn't build up. Ask me whatever else you need to know, not exactly sure what you need to know. Friend mentioned something about the master cylinder?

Thanks
If the master cylinder was working properly before, simply changing the pads would not affect that. But there are a few possibilities.

One would be that when you bled the brakes, you let the reservoir run dry, so you have a pocket of air at the master cylinder, meaning it needs to be bled/primed. Usually this requires a pressure bleeder or it has to be done on workbench, so hopefully that's not it. (You can try bleeding at the MC, put a rag in to catch fluid and use a flare-nut wrench to loosen (not remove) the fittings to the lines, then pump the brakes, tightening the fitting as you release the pedal, loosening as the pedal is pressed. Obviously you need an assistant for this. If you get fluid leaking out (and don't hear the air, it will be obvious if you're listening for it, like little pops) then the MC may not have an air pocket.

Two would be that when you replaced the pads, did you just press the pistons back into the caliper, or did you crack a bleeder screw to let the old fluid out right at the caliper? If the former, there is a chance you pushed contaminants and degraded fluid back up the line and have blocked an orifice or two along the way. A pressure bleeder may help (you can buy one online or you can make one, Kajtek1 posted a brief summary of that in the 210 forum although obviously it would be a bit different for the 126). However, it's possible you've damaged something that must be rebuilt or replaced; I had that happen a long time ago (which is why I always advocate to crack a bleeder screw rather than push the fluid back up the line).

Three would be that you're simply not doing something correct in the bleeding process and you have an air pocket in the line (so you see fluid, but the air is in the middle). If it's big enough, you could work a long time and never really get to it. Again, a pressure bleeder gets around this.

If you describe the EXACT process (break it down into steps) you're using to bleed the brakes, that might help as well.

Good luck!
 

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1991 300E24
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Discussion Starter #7
Thanks for the suggestions.

The way I bled the brakes is as follows; their were three of us bleeding the brakes. Myself pumping the brake, father topping up the master cylinder with fluid as required, a another guy who has one of those valve tubing things that lets you attach it to the caliper fitting and pump out the fluid without letting air back in. So as i pumped it out through the one way valve my father was topping up the mc, so I really don't think it has air within it. We did however only bled the rear brakes (if this matters?), but when we replaced the pads we also rebuilt the calipers, cleaned the pistons and fitted new seals due to the old ones being rotten.

There are no leaks anywhere as far as we can tell, we pumped the brakes many times to check, and we also checked the pressure? on all the brakes and it came out strong when the brakes were pumped. We also checked under the brake pedal and there was no fluid. Also while pumping the pedal I can hear air escaping?

The brake pedal itself is soft and doesn't need immense pedal pressure to stop, it just requires me to push it to the floor to get very small braking.

As I was typing this I got a text from my father who spoke to his mechanic and he rekons its the mc that has failed, so i'll probably end up replacing this and see if it fixes my problem.

I have a 1986 300SE :)

Thanks again,
 

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1982 300SD, 2001 f-150 4x4-348,000 passed tha benz!!
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You will need to bleed all of the calipers, as far as i know. Someone will chime in for the proper procedure. I usually start at the front right, then on to the back right, back left and front left, and has always worked for me. If you do manually bleed them be gentle with the brake pedal.
 

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With any gadget that connects to the bleeder screw you ALWAYS have the same problem. Air passes past the threads since you loosened the fitting and the threads are no longer tight. With 2 people do it the old fashioned foolproof way. Press down on the pedal, loosen the screw, let some fluid out (some prefer not to let the pedal go to the floor), tighten the screw and let up on the pedal, repeat.
 

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With any gadget that connects to the bleeder screw you ALWAYS have the same problem. Air passes past the threads since you loosened the fitting and the threads are no longer tight. With 2 people do it the old fashioned foolproof way. Press down on the pedal, loosen the screw, let some fluid out (some prefer not to let the pedal go to the floor), tighten the screw and let up on the pedal, repeat.
I agree with the above statement!

I've used a short piece of clear tubing that I put over the bleeder and into a container. This way you can watch for air coming down the line and for bubbles in the container.

I have a vacuum pump to bleed brakes and like mentioned above I've never liked it because of the leaking of air past the bleader screw!
 

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1991 420SEL, 1989 Porsche 928-S4, 1973 Lincoln Continental Town Car, 1960 Lincoln Continental Mark V
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There shouldn't be ANY brtake fluid in the car ("under the brake pedal"). You need to look along the path of the lines under the car - that's why I mentioned looking for puddles. The only part of the brake system that is (usually) inside the cab is the pedal, and the linkage to the booster.

If you're hearing "air" when you press the pedal, that's another indicator that you probably have a vacum leak (reference mramay posting). You shouldn't hear any air or hissing.

A master cylinder is cheaper than some of the other parts, but I'm not sure how likely a bad master cylinder would cause just poor pedal pressure. They usually show failure by leaks, oozing or gushing from cracked or failing canister, fittings, or hoses.

gregs210, just for my info, if you let the cylinder empty while bleeding, wouldn't you just have to add fluid and re-bleed properly? How likely is an air pocket to get "stuck" to the point of requiring a bench bleed?

The reason I ask is that I always assumed that if the bubble didn't "pop" backward into the reserviour when the pedal was pushed, then the process of bleeding would work it forward to one or more of the cylinders during the bleeding process.
 

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I've had the best luck bleeding the farthest first (R Rear on a LHD), L rear, then R front, then closest at L front.
 

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90 420SEL 240,000 miles strong
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bleed the brakes the old fashioned way. if that doesnt work, then I would point to your master cylinder. I had a 84 500sel which went bad. Didnt leak at all or anything, pedal would travel far and then hardly brake. Replaced it, and that fixed it.
 

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1966 200 Diesel; 1985 500 SEL (wrecked); 1991 420 SEL; 1979 280E (Home Market); 1993 190E 2.6
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MC's don't always show visually that they have failed. Many times its an internal seal thats shot and fluid just goes on by it when the plunger is actuated. Thats when you get the pedal to the floor, little to no braking, and no fluid loss set of symptoms.

Its a relatively inexpensive fix. I'd go ahead and do it if it was me and bleeding manually didn't make any change.
 
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