Need some expert advice:-
1986 420SEL, brake pedal suddenly went "soft" and almost full travel. Tried to bleed and flush system. Got very little fluid from either wheel, then pedal went to floor and have not been able to get it back since.
Changed MC, properly bled, installed and nothing changed.
No fluid loss evident at any wheel or component. Booster seems to be working when I turn engine on.
Again, Pedal goes to floor, yet no fluid loss . I am at a loss on this one.

Is it possible you have a defective master cylinder since your saying there is no air in the hydraulic system?

Did this start happening when you replaced the master cylinder?

Is it a new or used or aftermarket master cylinder?

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz

It is an aftermarket MC, which I am about to replace. I was not quite satisfied with the bench bleed. This problem surfaced before replacement of the MC. No recent brake maintenance was performed. Problem suddenly presented itself while wife was driving. She stated pedal travel was excessive, had to pump to stop but pedal was almost to the floor.
First attempt to bleed, showed little or no fluid at the wheels.
After that procedure, we lost all pedal pressure. Replaced the MC. No change. No fluid loss anywhere, and Booster is working. Do not believe this is an ABS problem, as I understand we should at least have front brakes even if the ABS fails. I will replace the MC again tomorrow, hopefully with a good bench bleed. Then try to obtain a unit for power bleeding of the system.
THX for your input.

Here's what I think has happened. The MC went out, (I would feel more sure of this if you said that there was fluid on the booster end of the MC when you removed it.) At that point you began bleeding using your wife as assistant and the engine NOT running. This means that you had very little pressure at the wheels when bleeding, so you ended up with air.

You THEN replaced the MC, but could never get it properly bled because again you bled with engine not running. Engine running will give you the pressure you need due to the booster helping things along. Some power brake cars require this and some you can get by without the boost. If you have a 300 pound weight lifter on the pedal, then you could probably get by without running the engine.

If you have indeed been bleeding with engine not running, you will need to start the engine and roll down the windows so that your wife and yourself can communicate over the engine noise and then bleed.

I've been there, done that with this whole scenario.

Good luck and Merry Christmas,

Thanks Larry,
This problem sure is elusive. No fluid was seen at the booster when MC was removed. Matter of fact, we saw no fluid at any visual point on the system. No fluid loss visible at the MC reservoir. MC was suspect as I had no problem pushing down the pedal without engine running, and of course effortless with the engine on. There should have been some resistance with the engine off.
Rebuilt M C is also suspect in that nothing changed after install. Bench bleed also did not satisfy me. Thus, I am replacing it tomorrow.
Probably going to consider power bleed of the entire system after successful bench bleed of the MC and installation. Nothing was done to the brake system at all prior to the first indication of pedal problems. All pads are in very good condition. This vehicle only has 35K on it. (Pretty nice for an '86). Mileage aside, I have had my share of problems surfacing due to deterioration of rubber hoses, etc. mainly on the vacuum systems. Only other problem was the ignition sensor mounted on the trans. (A real task to replace). I am sure with all the Forum fellows, we will lick this problem too.
Thanks again.

My goodness guys and gals. I have been smitten by the infamous resevoir tank dilema.
After reading numerous threads on the brake problems, one of the 'rude awakening' items refered to the resevoir and it's many partitons.
I have found that this is quite relevant, in that during the bench bleed I had to physically hold the MC off level, rocking front to back etc, to get the fluid into the rear partition. Then bench bleed went off without a hitch. After installation, it was paramount to monitor this tank partition (which is hidden if you are standing on the driver side of the booster), and required 'topping off' after each wheel was bled. The rear partition will go down to 'min' but the larger front partition will show 'max'.
However, what is wisdom, but to make mistakes and learn from them.
I am a much wiser and well informed SEL jocky again.
Thanks to all that helped.
Joe

While bleeding brakes, I have started by *always* filling the m/c reservior right to the neck. If the m/c is completely full, there is no chance there can be air in the rear chamber. You've figured it out this time, but it's always good to fill the m/c full to the top as a regular practice so you don't run into this again.

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)

I've never had a problem flushing my brake system with the engine off. If you open the valve when you order your "slave" to "push down" the pedal will easily go to the floor because the valve is open and the fluid will flow freely without pressure buildup.

THESE MERC M/C RESERVOIRS ARE TRICKY! It may appear to be full, but the back reservoir may be EMPTY! You have to look on the inside (side facing the engine) to see the rear reservoir level.

Also, I've found that I have to fill the res. to at least the bottom of the fill neck and then wiggle the res. back and forth sideways to get fluid to fill the rear res.

I alway siphon the front res., but the rear res. is tough to siphon. I don't have a tube small/flexible enough to reach it. I usually start by bleeding the rears while watching the inside on the reservoir to observe when the fluid gets to a minimum level. I have screwed up and sucked it dry. The sound is unmistakebale, so it takes a lot more bleeding to purge the air.

Once the rear res. is down to a minimum level, I siphon the front res., then fill with fluid and wiggle the res until I can see that the rear res. is full. Then I start bleeding and refill using the same procedure when both get to minimum level. After about three or four of these cycles and a quart of fluid, I am satisfied that the system is satisfactorily flushed.

Duke

I made up a cheap power bleeder with a $10 Walmart pump sprayer, a new MC cap and a tire valve stem. I only power bleed the brakes now and wish I had started decades ago. What a difference. One man job and the longest part of the job is removing and reinstalling the tires. Since we have four old MBs in the stable, the cap works for all four of them. Such a nice feeling to see clean, bubble free fluid running out of the tube at each caliper.

I've run into the same problem, and found that it takes forever to bleed the ABS equiped cars by pumping. I've switched to using the MitiVac to vacuum bleed, works fine. Hard to tell when you are air free by looking, as there is always some air coming through the threads on the bleeder, but the pedal firms up quite nicely.

Also good for changing brake fluid since you can see the color change.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

That's the "problem" with vacuum bleeding. If you remove the bleeder valves and wrap the threads with teflon tape, it might seal them enough to not suck in air.

Duke