Can't get fluid to rear brakes - PeachParts Mercedes-Benz Forum

Rudebutler · #1 10-21-2007, 12:43 PM

I had the rear metal brake line go on my 83 300D. Replaced rear lines, pads, rotors. All seems good. Except I can't get fluid to the rear brakes. The car is slightly tilted back, reservoir full to cap.

Even when I pull the rear flanged line to the master cylinder nothing comes out. When that line is pulled ans someone steps on the brake pedal, shouldn't something come out? Is something wrong or am I missing something here?

t walgamuth · #2 10-21-2007, 01:00 PM

It must be plugged or a failed master cyl.

Tom W

bustedbenz · #3 10-21-2007, 01:02 PM

I'm not trying to insult your intelligence.

But are you sure you properly filled both the front "chamber" and the back chamber of the brake fluid tank under the hood?

My dad and I changed a master cylinder that we suspected was bad because we were trying to bleed the brakes and couldn't get anything to go to the back. Turns out, we just didn't know how to fill the reservoir. There's a plastic wall in the tank that keeps fluid from filling the back section until after you have filled the front up to full... you have to overflow the front section until the back is full. Why they did this... if anybody knows then please tell me.

Rudebutler · #4 10-21-2007, 01:02 PM

any way to unplug it?

Rudebutler · #5 10-21-2007, 01:20 PM

No offense taken. I'm not sure if the res. is full to be honest-the new fluid is so clean compared to the old it's hard to see it in there. Does the back section feed the rear brakes? The line off the master cylinder is to the front. How can I be sure it's full?

When the brakes are pumped, the bleeder line pulls and pushes fluid from the jar I'm using. I'm thinking that's a good sign for the master cylinder, no?

bustedbenz · #6 10-21-2007, 01:24 PM

Don't know much about it. Maybe it was the fronts we couldn't bleed. It's been a year and I can't remember.

It was just my first thought. Let the brake experts tell you

Wodnek · #7 10-21-2007, 01:55 PM

Quote:

Originally Posted by bustedbenz

I'm not trying to insult your intelligence.

There's a plastic wall in the tank that keeps fluid from filling the back section until after you have filled the front up to full... you have to overflow the front section until the back is full. Why they did this... if anybody knows then please tell me.

They did this so that if you have a break line rupture you can only lose the fluid on that half of the master cylinder. That way you will still be able to stop the car.

toomany MBZ · #8 10-21-2007, 05:20 PM

The MC is chambered inside, is hard to tell, with just one fill opening. New lines should be clear, if you can, rig up an air hose, try blowing through. There should be fluid coming out of the bleeder screw, but there is much air in the new lines, so I imagine it would a VERY long time to fill.

Rudebutler · #9 10-21-2007, 10:44 PM

I really think both chambers are full. And it would make sense that the front chamber fills the rear lines since the the rear line connects to the front end of the master cylinder. Does anyone know?

This all started about a month ago when my rear brakes froze. Pulled over and they released on their own, only to find out that the rear metal brake line ruptured. (It was quite rusty). I got home thanks to the 2 chamber system, no problem

I have read in threads that rust and such in the lines can cause the calipers to freeze. But, could it have been caused by a faulty master cylinder instead? I, sadly, am ignorant of the inner-workings of a brake master cylinder. Could the piston not be returning to release pressure on the rear line?

Is there a way to test the master cylinder? Clean it? I'll spend the money on a new one if needed, but would rather not if it's not the problem.

kerry · #10 10-21-2007, 10:48 PM

If you have no fluid coming out of the fitting where the rear line attaches to the master cylinder, the problem is definitely associated with the master cylinder. I wouldn't look any further back into the system until you get fluid coming out of that fitting on the master cylinder.

TTCummins · #11 10-22-2007, 08:13 AM

I dont know how you replaced your line, if you went all the way from the master cylinder back, or if you cut it at a place were it was good and replaced the bad. If you cut it, I have seen that when using a dull tube cutter the wheel will more or less just narrow the tube down to nothing when it cuts it off, if you do not deburr it the line will be pretty much pinched shut making much of the symptoms you are describing. I don't know if you have abs on the car or not, but I work daily at and abs engineering company and i can tell you there can be problems with that as well, IF you have one.

TTCummins · #12 10-22-2007, 08:18 AM

OOO one other thing, if that section of the master cylinder ran dry during the repair, they can be VERY difficult to get fluid to come back through the port. The best way to do it is have somebody get a mighty vac pull a vacuum on the rear calipers crack the bleeder, and have somebody pump the pedal just to actuate the cylinder. I had the same problem on a few master cylinder replacements i have done, spending hours trying to bleed them after i had them all mounted up.

vstech · #13 10-22-2007, 08:50 AM

you may need to get a "bench bleed" short length of brake line and hook to the MC and put the line in a fresh tank of brake fluid. with the MC full to the top, and the lines in the can, pump and pump. when the air bubbles go away, you have bench blead it then you can move on to bleeding your new line.
another way to go about this, go to a parts house or junk yard, and get another cap for your MC drill a hole in it, and screw in a fitting, hook up a standard pump up sprayer to the cap, then fill the sprayer with a couple quarts of brake fluid, now just put the "sprayer cap" on the MC and pump a few times, walk around to the bleed screws and just let the fluid flow.

Rudebutler · #14 10-22-2007, 10:31 AM

Quote:

Originally Posted by TTCummins

I dont know how you replaced your line, if you went all the way from the master cylinder back, or if you cut it at a place were it was good and replaced the bad. If you cut it, I have seen that when using a dull tube cutter the wheel will more or less just narrow the tube down to nothing when it cuts it off, if you do not deburr it the line will be pretty much pinched shut making much of the symptoms you are describing. I don't know if you have abs on the car or not, but I work daily at and abs engineering company and i can tell you there can be problems with that as well, IF you have one.

I replaced the entire line with a pre-fab from Fastlane. Installation went well, I have no reason to think it was pinched anywhere. I don't think it has ABS. Do '83 300D's have ABS? I've only had the car a year, still finding things out

I wouldn't doubt the master cylinder went dry in that section as the line was off ripped in 2 and I did have to drive it a little before the repair. I will definitely try the bench bleed. I am familliar with the homemade power bleeder made with a sprayer.

Thank you all for the advice! I'll post after I try these things.

Rudebutler · #15 11-02-2007, 03:56 PM

Well, I tried all those things and no luck. Removed the master cylinder and tried to bench bleed. No love there, either.

Purchased a new master cylinder and all is well. The old one was obviously fried. And that's actualy good because it explains why the rear brakes locked in the first place, setting off this wonderful chain of events that has drained my bank account for at least 3 bills.

Thank you everyone for your help! The folks on this forum are the BEST!

PS-The brakes don't squeal anymore. They had since I bought the car..I kinda miss it. Honest, I do.

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