I'm working on my 1990 300D and i was using a mighty vac to bleed the brake calipers. The problem is the left rear caliper. I was only able to pull a little fluid out of it and now no fluid will come out. I took the hose off thinking it was clogged, but it wasn't. I can't get any fluid to come out of the steel line with the hose off either. I even tried with the car running. I also noticed before i tried to draw the old fluid out of the calipers the rear brakes did not stop turning untill i pressed real hard on the peddle. Both rear wheels where off the ground when i put it in drive and tried to stop the wheels.Prior to that the brake peddle was hard and did not go to the floor. Now it just goes to the floor, because air got into the system, but i still can't get any fluid to come out that line. Any body know whats going on there? I searched 17 pages of threads and can't find nothing on it

I'll bet you got some air in the line. I just changed the fluid in my 560SEL yesterday and I somehow got some air in the line when bleeding the left rear and I got the same symptoms you are describing. Just pump up the brakes for a bit and bleed the air out of the line the normal way. Worked for me.

I've never really been successful with the Mityvac.

I therefore go back to bleeding the brakes in the traditional manner, with one shortcut:

I get a tall bottle (like a horseradish bottle) and put a bit of fluid in the bottom. Then get a hose that fits the bleeder screw on the RR wheel. Put one end of the hose on the screw and the other end in the bottle. Stand the bottle up carefully on the pavement (the hose must be just the right length).

Open the bleeder screw 1/2 turn.

Get in the vehicle and slowly press the pedal down and let it all the way back up.

Repeat 10 times.

Return to the wheel and close the bleeder screw. Remove the hose.

Add fluid to the reservoir (make sure that the rear reservoir is filled). It's happened to too many of us that the rear reservoir, which controls the rear brakes, goes dry because it is much smaller than the front reservoir. A casual observance of the reservoir will make you think everything is fine, when actually, you are looking at the front reservoir only.

Repeat for the LR wheel.

Repeat for RF wheel.

Repeat for LF wheel.

You are done.

I personally would never bleed brakes with a mityvac. Now that there is air in the system use the two man system with the engine running so that you have the brakes boosted. You will then get out the air.

Good luck,

That bears repeating. I spent all day fooling with a 190E before discovering that you have to fill it all the way up into the neck before fluid goes into the rear chamber.

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Rick Miley
2014 Tesla Model S
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Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
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Actually, it's more complicated than this. If you fill it up close to the top, but not high enough, the fluid won't spill over the barrier into the rear chamber.

If you fill it too high, right up into the neck, the fluid will trap an air bubble in the rear chamber and you still can't get the rear chamber to fill.

So, you must carefully fill it to just above the height of the barrier, but not so high that the fluid enters the neck. Then it will spillover and fill the rear chamber very nicely. You will need to continually add fluid, very slowly, to maintain the perfect fluid height while this process is occurring.

It's easy, once you do it the first time.

I don't think your problem is air. If you can't get any fluid to come out of the steel line, you either have a blockage or a pinched line. If it was air, you would get fluid and/or air to move through the line. Since you have to rebleed the whole system anyway, isolate the left rear line (where it tees off with the right) and see if it's clear from there to the caliper end (blow air through it). If the right rear's getting pressure you know the line is good up to the tee. If the right rear isn't getting pressure say so. Check the steel line carefully for crimps. Once you know your lines are good, abandon the mityvac method and bleed them old school. RR the LR then RF then LF.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

The typical problem a first timer encounters when performing this task is running the reservoir out of brake fluid. The reason he can't get any fluid out of the back is because there's no fluid in the portion of the reservoir which feeds the rear lines. BTDT. See the previous advice on filling the reservoir.

- JimY

Exactly.

OK, if you say so. But not getting any fluid from the lines when there is none in the reservoir is pretty straightforward...and it doesn't explain the caliper behavior he described or the exessive pressure required to engage the rear brakes ...

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

Good point there, I didn't address that issue. On a 15 year old car, the first thing to address is the flexible brake hoses. They swell internally as they age. Usually they end up acting as a one way valve, keeping the caliper engaged after releasing the brake pedal. However they could be limiting pressure to the calipers. I'd also recommend inspecting the metal brake lines along the length of the car. Insure there is no damage - no crimps, pinches, etc. If that doesn't fix it, I'd start looking at the master cylinder. Perhaps the piston for the rear brakes has frozen? This was a common failure mode on earlier MBs - 123 chassis especially. But again, it generally resulted in the rear brakes remaining engaged after releasing the pedal. BTDT myself on that one. And I've never heard of that problem on the 124 - it uses an aluminum bodied master cylinder, as opposed to the iron body on earlier cars.

My rambling $.02,

- JimY

I found the problem and it was i didn't have the master cyclinder full. I would have never guessed that. The other problem was the rear brake pads were frozen and not moving. I took them out, cleaned them and the caliper and now they slide free. Thanks for all the advice, and i' did end up bleeding the brakes the two man way, old school. Thanks again.

Good call jcyuhn! And thanks jesse 56 for posting the soultion. So many threads here are abandoned before resolved...this one may actually help someone down the road who is reading it after searching...

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

like me... I'll be bleeding the wagundy's brakes tomorrow and I had questions... huzzah for concluded threads.

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'82 300TD
'77 6.9
'75 280S
'74 280
'87 Porsche 944 turbo