Perplexing brake issue would appreciate advice - Mercedes-Benz Forum
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post #1 of 23 (permalink) Old 06-11-2019, 02:01 PM Thread Starter
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Perplexing brake issue would appreciate advice

Last year I replaced 3 of the brake calipers, 4 sets of pads, rotors and hoses on my 1986 560sl. The fourth caliper was backordered and I didnt receive it till mid winter. The brakes, although I wouldnt say they were 100%, did an excellent job of stopping the car. Drove around all May no problem. Pedal was slightly lower than I had expected but acceptable. Sooooo, nice Spring day I thought I'd replace the 4th caliper and all went well till I tried bleeding it. Couldnt get flow. Tried power flush, Mighty vac and finally the old fashion hose in a bottle. Did get a slight amount of fluid when I reduced the size of the tubing to 1/4' od but pumping the pedal like crazy produced very little flow. Tried the other wheels and got similar (but slightly better) results. Was hearing some squeeking under the dash so I ordered a new master cylinder. It was next on my list only because it did a peculiar thing the first time I engaged the brakes. Only happened as I left the garage and drove down the driveway. The brake pedal would suddenly drop an inch. Braking was fine and it would never do it again till the next time/day/week that I drove the car. Installed the new master cylinder (bench bled, etc) and have a nice and firm pedal. Start the car and the pedal goes to the floor. Pump a number of times and it firms up but if you leave the your foot on it soon sinks back. Shut the engine off, pump a couple of times and great pedal. I went through a number of websites on diagnosis and still am not able to determine if now (the only thing left) the booster is bad. I dont have hard pedal as they suggest (when its running), I disconnected the vacuum line to the canister and applied vacuum with the mighty vac and got it up to 10 inches (and it held) before my hand got tired of pumping. The check valve works and I dont see any leaks in the line from the manifold. When the engine is running I dont notice any tendency to stall or hear any hissing. I do notice that when the vehicle is running the economy (vacuum) gauge on the dash moves off the peg when I'm pumping the brakes and goes back when I stop. Sorry for the length but I didnt want to waste anyones time speculating on things I've already tried to diagnose. But any advice would be gladly appreciated. I really, really want to make sure before I plop down serious money for a new booster.
Thanks in advance
Bob
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post #2 of 23 (permalink) Old 06-11-2019, 04:22 PM
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I'm not sure about an 86',but some cars have a shuttle valve that divides front and rear brakes. If fluid pressure drops and it goes to one side it needs to be centered again to allow fluid to the other side. Like I said,not sure this applies but thought I'd throw it out there.
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post #3 of 23 (permalink) Old 06-11-2019, 04:53 PM Thread Starter
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If that were the case how would you know and how would you fix it?
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post #4 of 23 (permalink) Old 06-11-2019, 04:54 PM
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Since your car has ABS, don't you need to have the key in and turn all the way to the right (accessories powered) in order to be able to bleed the system?

Also when filling up the master cylinder, you will need to fill it up to the tippy top. Both the front and rear chambers are separated so if one hoses or calipers goes bad you won't lose all of your braking abilities.

Also, paragraphs. Please use them
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post #5 of 23 (permalink) Old 06-11-2019, 04:58 PM Thread Starter
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I’ve never had the key on or turned the wheel in all the years I’ve owned this vehicle. Always bled fine. For sure on the tippy top. Sorry about no paragraphs- hard enough to type this on a phone. But I’ll try next time
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post #6 of 23 (permalink) Old 06-11-2019, 06:03 PM Thread Starter
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Red face

I'm still trying to wrap my head around what would be the consequences if the little spool valve inside the booster had a seal failure. If it allowed atmosphere into the front (vacuum ) side of the bellows wouldn't it cancel out and wouldn't it function as if there was no power brakes? Hard to press? If the normal position of the diaphragm is vacuum on both sides ( without your foot on the pedal as if you were driving) then conditions are the same on both sides or equilibrium-so no movement. Pressing on the pedal causes the slide valve to allow atmosphere into the back while closing off the front to keep its vacuum causing unequal pressure so the diaphragm moves forward assisting the braking effort.
So how can I have firm pedal with engine off-no pedal with engine on?
Paragraphs included for Beast
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post #7 of 23 (permalink) Old 06-11-2019, 06:17 PM Thread Starter
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Trying not to monopolize this thread but I've been reading all day and many of the booster/brake rebuilding outfits have great sites with troubleshooting PDFs. They all address fading pedal as being a hydraulic issue and a hard pedal (I assume with engine on) as a vacuum (booster/check valve/hose) issue. Same thing with low pedal-air in hydraulic, bad lines, bad hoses, brake shoes out of adjustment??? Sigh!
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post #8 of 23 (permalink) Old 06-11-2019, 06:50 PM
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I had something else written about the booster before I swapped it with the key mention.

Did you observe any fluid in the booster chamber? You mentioned the pedal liked to sink but still had some push to it. I had a similar symptom when my master cylinder failed and let fluid into the booster chamber.

I would pull the master cylinder off again and check inside the booster for fluid. If you have fluid, then you should replace the booster. The FSM gives an allowance of 100CCs of fluid to enter the booster before the booster seals have been compromised. However, if you've been on any sort of incline then the fluid will have touched the seal and ruined it.

Regardless, I would look at your booster. I typically don't rebuild anything since it's easier to just go to NAPA Autoparts and pick up a reman one there. My Booster is a reman'd one from them and has been working admirably. No issues since I rebuilt the whole dang thing.
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post #9 of 23 (permalink) Old 06-11-2019, 07:02 PM Thread Starter
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There was a small amount of fluid in the booster which I washed out with 90 percent rubbing alcohol and the dried. That would have indicated the seal at the back of the master was allowing fluid past it. If I installed a new master that should have taken care of that. Who’s rebuild was NAPA offering? Would it be a Girling core?
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post #10 of 23 (permalink) Old 06-11-2019, 07:04 PM Thread Starter
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Also beast what model/year was it for?
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