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Braingears
Expert
Date registered: Apr 2006
Location: St Petersburg, FL
Vehicle(s): G320 & ML320
Posts: 1450
| Rear Brake Problem
I had to replace the rear brake cylinder on my G-Wagon (thanks to EuroTruck who had the parts in stock). While bleeding the brakes I discovered another problem. I am having something that is either sticking or blocking the rear brakes from getting fluid properly. When I press on the brakes with a strong amount of pressure, I can feel a "release" in the the pedal, which them allows the rear brakes to work. It is effecting BOTH of the rear drum brakes. If I am correct, both of the rear brakes are operated from a single brake line. While diagnosing the problem, I bled all of the lines to and from the ABS. At the ABS unit, I was getting the proper amount of flow and pressure while bleeding them. While at each of the rear brakes, I had them WIDE OPEN and could not get a drop out of them. To test an idea, I started the engine, then pressed the brake pedal as hard as I could, then "release". I was then able to get fluid to pump through the rear lines. It felt like a sticky piston or something. Once the pedal is depressed and allowed to come back up again, the brakes do not stick in any way, and the wheel free-spin as normal. I remember reading that there is a leveling unit that can control how much brake pressure the rear brakes get. Can this unit be causeing the problems? What else should I be checking? |
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EuroTruck
Extreme Veteran
Date registered: Apr 2006
Location: Oakwood, Georgia - USA
Vehicle(s): 2012 Audi A4 Avant S-Line, 2015 Ridley NOAH SL
Posts: 518
| RE: Rear Brake Problem
Chuck, I would definitely take a look at the rubber line that feeds the rear brakes. If it is swollen on the inside, it can certainly cause trouble. To check the brake bias valve, pop the actuator linkage off of the top of the rear axle. You'll see the linkage attached to a ball socket on the top of the third member of the rear axle. The valve itself is #5 in the diagram and #50 is the linkage rod. Once the linkage is loose, move the lever #11 down to see if the valve is free and not frozen in any way. If lever #11 is pushed up as far as it will go, the valve is then allowing for the maximum amount of bias towards the rear brakes. At this point, you will get the maximum flow of brake fluid to the rear brakes. Try to bleed them again and see what happens. I'm curious to see what you find. -Sean
Edited by EuroTruck 4/8/2007 10:34 PM
 (W463 ALB Valve.jpg)
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 W463 ALB Valve.jpg (22KB - 5 downloads)
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Jonathan Joseph
Expert
Date registered: Oct 2006
Location: Charleston, South Carolina
Vehicle(s): 2004 G55
Posts: 1538
| RE: Rear Brake Problem
Chuck,
When you say you were pressing and then releasing, just to clarify, are you closing the bleeder screw before releasing? If not you'll be sucking air back up the line when you release the peddle.
Jonathan |
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AlanMcR
Expert
Date registered: Apr 2006
Location: US, CA, Los Altos
Vehicle(s): G300DT E300DT 230SL
Posts: 3500
| RE: Rear Brake Problem
Your truck should not have a leveling unit. That function is now called EBV (Elektronic Brems Verteilung) and is essentially a software program in the ABS computer. It is easy to identify the brake valve, if you have one. There will be a spindly arm attached to the drivers side of the read diff.
On to diagnosis, you might want to remove the rear flex hose and see if there is fluid at that point. Flex hoses fail in two ways. I just caught the "wears and cracks open" variety on my G. My 230SL suffered from the other variety, where the lining of the tube swells up and occludes flow. Usually the latter problem is discovered when you find yourself stuck at a light with the brakes locked up. The pressure of the brake booster is able to get the fluid through, but the return springs are not strong enough to return the fluid. Of course, your ABS unit could be bad.  |
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Jonathan Joseph
Expert
Date registered: Oct 2006
Location: Charleston, South Carolina
Vehicle(s): 2004 G55
Posts: 1538
| RE: Rear Brake Problem
Perhaps I misread your post, maybe you're saying that "upon" applying hard pressure there is initially resistance and then a "release" ? What amount of peddle travel do you experience after this release? |
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EuroTruck
Extreme Veteran
Date registered: Apr 2006
Location: Oakwood, Georgia - USA
Vehicle(s): 2012 Audi A4 Avant S-Line, 2015 Ridley NOAH SL
Posts: 518
| RE: Rear Brake Problem
AlanMcR - 4/8/2007 10:35 PM Your truck should not have a leveling unit. I thought that as well Alan, but when I ran his VIN through the EPC, the valve came up as a viable part number for his truck. That should be taken with a grain of salt though, as the EPC does not always get it right. It keeps us on our toes.  |
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Braingears
Expert
Date registered: Apr 2006
Location: St Petersburg, FL
Vehicle(s): G320 & ML320
Posts: 1450
| Re: Rear Brake Problem
I do think that I have a leveling unit. I remember seeing the linkage rod (50) connected to the rear axle.
When I say "release", it feels like when I give the pedal a lot of pressure (30lbs - 60lbs), it forces fluid through the valve in question (if that is actually the part giving the problem). It feels like a check-valve of sorts. Then the brakes instantly grab.
What is the actual full function of this check valve? If I disconnect the linkage rod (50) and zip-tie it so that the valve is in the wide-open position, will it cause any problems?
I will try what Sean recommended and see if it feels unusual through its movement range. |
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Braingears
Expert
Date registered: Apr 2006
Location: St Petersburg, FL
Vehicle(s): G320 & ML320
Posts: 1450
| Re: Rear Brake Problem
I hopped under the truck and reviewed the mechanism. The arm (11) and Spring (47) are both moving properly. The spring is more like a tentioner to the valve (5).
When the axle is weighted down, it applies more spring pressure to the valve (5). The valve did not seem to have any movement in it. The spring pulled flap on the valve (I guess this would be the wide-open position). When I release the spring from pulling on the flap, it did not push back or return to another position.
I do not know how this valve works. I do not know if the brake pedal creates the backwords pressure against the spring or what..
Other than the flap which I can guess pushes on a valve piston, the valve did not push this piston back as if it was spring loaded or anything.
Any recommendations? |
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DesertStar
Expert
Date registered: Apr 2006
Location: San Diego, CA USA
Vehicle(s): 85-280GE/95-G320/08-G500
Posts: 2156
| RE: Rear Brake Problem
I wish I could help more, but I think the ABS may add a new dimension to the equation. I had to replace my cylinders and experienced same as you. The rear brake valve when no load on G doesnt let much pressure to the cylinders. A loaded G will experience more rear braking...naturally, as the load forces lever on axle to push up on valve and open for more pressure and when stopping quick, a rise in the rear of the G body will of course release the valve a bit decreasing rear axle braking to prevent skidding. Kind of clever really, but I do not know how ABS system works with this setup.
Mike |
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Braingears
Expert
Date registered: Apr 2006
Location: St Petersburg, FL
Vehicle(s): G320 & ML320
Posts: 1450
| Re: Rear Brake Problem
I disconnected it this morning and tip-tied it tight. I had a lot less problems with my brakes today.
I will have to check on this part and see about replacing it or bypassing it completely. |
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