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#1
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126 Sinking brake pedal
I have gone through the search section to resolve the sinking pedal issue . Like to hear from those who successfully rectified this fault .
Thank you mak 300se 89
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mark |
#2
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1. Master Cylinder
2. Booster 3. Worm brake lines(rubber) Booster check - depress brake pedal - hold it there - no need for extreme pressure - while holding brake pedal down - start car - brake pedal should fall thru a bit - if so - booster likely OK. Look at brake hoses - any gettin' fat? - no? - next line Master Cylinder
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Mike Murrell 1991 300-SEL - Model 126 M103 - SOHC "Fräulein" |
#3
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Just went thru this on my 86 300sdl. It was the master cylinder. Even though I had no fluid leaking anywhere, it was the master cylinder. I stubbornly replaced all my rubber brake hoses first as they were 24 years old and needed replacing, but my pedal still would sink slowly while sitting at a red light. I replaced the master cylinder and all is well.
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John 1986 300sdl 1985 380se |
#4
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Booster problem will exhibit themselves as very firm pedal, hard to press down.
Master problems will show by the sinking pedal. Hose problems will show by squishy pedal. I THINK. I too am working through some brake pedal issues. First up is replacing the soft lines. Mine are stainless steel lines that have been on there since I bought the car so 5 years + however many before that. Potentially another 5-7...
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out. Check here for compatibility, diagnostics, and availability! (4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!) 82 300SD 145k 89 420SEL 210k 89 560SEL 118k 90 300SE 262k RIP 5/25/2010 90 560SEL 154k 91 300D 2.5 Turbo. 241k 93 190E 3.0 235k 93 300E 195k |
#5
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Well, if there are any leaks that would be the cause. And, you will find your fluid level slowly going down. If there is not a leak in the lines somewhere, then your master cylinder is most likely bad. I had this happen in one of my 300SDs, fluid was leaking into the brake booster. IF there is fluid at the bottom of the brake booster, replace both the MC and the booster, as brake fluid is extremely corrosive and will eventually cause the BB to rust. Used BB is the way to go in that case, as new they are very expensive! Replacing the MC - hour or 2, BB - an hour at most. Bleeding and making sure there are no leaks - a few hours, be meticulous!
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TC Current stable: - 2004 Mazda RALLYWANKEL - 2007 Saturn sky redline - 2004 Explorer...under surgery. Past: 135i, GTI, 300E, 300SD, 300SD, Stealth |
#6
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new MC
When i had the sinking pedal problem on the 300se ,the MC was duly replaced . its still the same ,slow sinkage at the lights . i suspect the ABS unit may be harbouring a pocket of air bubble, though i had pressure bled the system .
is there a bleed procedure for the pump Thanks mak
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mark |
#7
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The ABS pumps have accumulators, spring loaded cylinders holding brake fluid, inside. Depress the brake pedal 60 or 70 times to relieve the pressure. Then bleed the brakes.
I also recall a page in the service manual detailing a brake bleed screw on the pump body. I forget the procedure.
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Regards Warren Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL ENTER > = (HP RPN) Not part of the in-crowd since 1952. |
#8
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Bosch ABS2, which is what our 80s non ASR 'benses have, do not have pressure accumulators. Just a return pump and valves. ASR equipped W126s do.
On non ASR cars bleed the brakes as you would normally. There is no extra bleeding needed to clear air from the ABS system. The problem here is probably the master cylinder, but it might be possible for the ABS system to cause a sinking pedal if it were broken somehow. Unlikely though. -J
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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket Tesla Model 3. 205,000 miles. Been to 48 states! Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels. 2014 Cadillac ELR 2013 Fiat 500E. |
#9
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Quote:
If you catch it soon enough, you can flush out the brake fluid accumulated in the booster and "save" the booster. Jim
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14 E250 BlueTEC black. 45k miles 95 E320 Cabriolet Emerald green 66k miles 94 E320 Cabriolet Emerald green 152k miles 85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles |
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