87 300D -no ASR- I have suspected air in the lines so I attempted to begin bleeding from the right rear first. Method: depress brake pedal - loosen bleed screw - pedal go's to floor - keep pedal to floor and tighten bleed screw - release brake pedal from floor - keep brake fluid topped off in reservoir - repeat until no air, just fluid - tighten, go to next wheel.

After the first pump of pedal, and release of first fluid, I could not get any more fluid to come out. The fluid reservoir tank on top the master brake cylinder never dropped level to add anything!

So I tried bleeding the front and the only fluid I could get to move through was from the second stage of the master cylinder only. The first stage just go's to the beginning of the second stage and without much if any resistance picking up no fluid from the reservoir!

What am I doing wrong?

Quick update:

Just read my 124 bible and it confirms my method as proper. I'm beginning to suspect my master cylinder may need attention. But why would I have had a working first stage before, but not now? It's never enguaged the second stage before. Now my only braking is in the second stage, pedal almost to the floor? BB

Sometimes junk can block the nipple, thus preventing fluid from coming out as it should, especially if it has been a long, long time since the car had a fluid flush. What was it that caused you to suspect air in the lines? If you aren't getting any fluid out, you may not be getting any pressure by pumping the brake pedal? Normally, you can get the fluid to flow by gravity. Try removing one of the nipples completely and fishing a thin wire gently into the hole to see if there is crud blocking the hole. Also check the nipple once it is out for blockage.

Steve

I had something similar happen on a '68 Plymouth once. What happened is that some crud actually managed to block the hole where the master cylinder sucks fluid from the reservoir. It seems like this would be less likely on a Benz, but still possible. Since the master can't take in fresh fluid, it just allows you to pump out the fluid that's in the lines. When that's gone there's nothing but air. Try pumping up the brakes and cracking and closing the bleeder like normal, but then let up on the brake. There should be vacuum in the system if the master is blocked off from the reservoir. Open the bleeder again and listen for a sucking sound, or put a tube on the bleeder and place the other end in a container of fresh brake fluid and see if it sucks any fluid into the system.

Note: I have no experience with ABS, so I don't know if it can cause a problem like that.

I have now tried both rear brake cylinders, same issue! I took each bleed nipple out, cleaned, and returned. The system has been flushed three years ago and have desided it's time again, late 2 years!

I thought I'd try bleeding first to see if in fact there was air before proceeding to flush the whole system with new synthetic fluid. But now I'm lost!

Brakes are not as good as they should be. Although adiquate, I have always had to push harder on these brakes than I thought one should have to. Although they stop straight without pulling one side or the other, they are no where near as good as in my 190E, or even the 450SL, which needs all new pads, discs, etc. BB

Although this car does have ABS, which according to what I've read shouldn't make any difference, it would if I had in addition, ASR (automatic wheel slip control)

Yes, you described what seems is screwy with this one, like the master cylinder plunger is not picking up fluid, just pressurizing the lines, but after fluid is in the lines pushed out, and I was able to get that much out, why is it not replacing it?????

By the way, welcome to the forum Steve............. BB

The hole that the fluid passes through to go from the reservoir into the master cylinder is pretty small, and can get clogged with crud. If the hole is clogged, no fresh fluid can get through. If you have acces to an air compressor you can try blowing air through the system backwards (from the wheels). That's what worked on the Plymouth. If not, you could try removing the reservoir from the master and cleaning it out manually.

By the way, thanks for the welcome! I've been lurking for a while, but haven't had a reason to post yet.

I've bled 124 brakes many times. Your reservoir is divided between the front brakes and rear brakes. The front 90% or so of the reservoir feeds the front brakes, the remainder the rear brakes. It's easy to run the small rear portion of the reservoir dry when bleeding the rear brakes. The filler neck of the reservoir feeds only the front portion. You have to fill the reservoir until fluid is in the filler neck before fluid overflows into the rear portion. You'll be able to see bubbles from the back to the front of the reservoir when this occurs. Shine a bright light right up to the reservoir to see how full the rear portion is.

I learned all this the hard way. My first time doing the brakes on a 124 was quite frustrating.

I've found it isn't necessary to close the bleeder valve after every pedal stroke on a 124. I've had very good results placing a tube on the bleeder into a cup of brake fluid, opening the bleeder, and pumping away. For whatever reason the fluid doesn't seem to flow back up into the caliper, but rather is sucked down from the master cylinder.

You'll need to top up the small rear portion of the reservoir two or three times to thoroughly bleed the rear brakes.

More than you really wanted to know...

- JimY

If you like to flush your brake system regularly, you might want to try something like these,

http://www.speedbleeder.com/

I've no experience with them but they look cool.

I do have this and it actually works,

http://www.aep.bigstep.com/eezibleed1.htm

and following a link on that page I found this,

http://www.bmw-m.net/TechProc/bleeder.htm


HTH
Michael

__________________
Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

I have bought a MityVac and hope to use it, but not until I've corrected whatever it is that is amiss.

I really think now that my master brake cylinder's primary stage is likely worn and only pressurizes braking action now in the secondary, or emergancy back-up stage.

I am going to take the master out and have a look at the piston seals and cylinder for corrosion etc. The outside condition looks brand new and is deceiving! BB

No, no, no. Didn't you read jcuhn's post? The only thing wrong is that you ran the rear portion of the master cylinder dry. Fill it up all the way into the filler neck and then you'll see it. Once you get fluid in there it'll be fine. I did the same thing on a 190E. Just about everybody does the first time they do Mercedes brakes. I can't believe it took so long before somebody said this.

__________________
Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

By pushing you brake pedal to the floor (or letting it drop there), you run a significant risk of ruining the seals in your master cylinder. You normally never would push the pedal all the way down, so deposits form at the back end of the master cylinder bore over time. You ruin the piston seals when you force them over the deposits by bleeding this way.

When bleeding, you should first see how far the pedal goes down under normal pressure, then place blocks of wood, or whatever, underneath it so it doesn't go past this point when you bleed the brakes.

I hope you didn't damage anything, but I wouldn't be surprised if you need a new master cylinder in the near future.

GregS

Mityvac doesn't work on cars with ABS. Two atmospheres of pressure are required to bleed the brake system (per the shop manual), a mityvac causes less than one atmosphere of pressure...

The comment by GregS is appropriate. Don't go all the way to the floor with the pedal when bleeding. Half way is sufficient. I don't think there's much risk of ruining the seals in the master cylinder on these cars. They use an aluminum master cylinder, so it doesn't collect up as much seal destroying junk as cars with a ferrous master cylinder. Still it's always good advice to limit brake travel when bleeding.

- JimY

While I don't doubt you can raise the dust so to speak with this method, that is if there is a lot of dust, or tiny abrasive particles, a clean system relatively free from debris should not be compromised doing such.

Your only moving the plunger a small distance beyond, but if there is a ridge formed at where it normally stops, that ridge may do more damage than anything else!

I just ordered a new MBS, there just under $90. It should be here early next week via UPS. BB

More of a learning curb than expected...........

I've actually thought about the use of my portable air tank and sucking air out to make a vacuum. If I connected this up to my reservoir, could I suck out the fluid?

Although my current fluid is quite clear, it has been 3 years in there, and I do want to replace with synthetic anyway. BB