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86 300sdl Brake pedal sinking at idle
Good evening all,
a new perplexing problem with my 86 300sdl. First of all I have new rotors, brake pads, sensors, and rubber brake hoses. Fluid level in the master cylinder reservoir is normal and not changing. Brakes work excellent EXCEPT occasionally at idle at a red light my pedal will sink slowly to the floor almost. At first I thought master cylinder, but no leakage and no fluid level drop. In my driveway with the car off, I can pump the brake pedal two or three times to release all of the built up vacuum in the booster and the pedal sits there hard as a rock. So I am thinking not the master cylinder. Today at a red light it did it again so I pumped the brake pedal 4 or 5 times and the pedal comes all the way back to normal, but I hear a whooshing sound at my feet. Could my booster be leaking? Could I have a vacuum leak somewhere? My transmission shifts perfectly so I was ruling out the vacuum source for it, BUT... a Climate control vacuum leak? Could that affect your brakes? Thanks in Advance for all comments.
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John 1986 300sdl 1985 380se |
#2
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ok.
if you come to a stop, and hold your foot lightly on the brake, and the pedal drops to the floor, your seals in the MC are shot and you need to either rebuild yours or put a new/rebuilt one in. period. sometimes they will leak internally and not lose any fluid. sometimes they will leak into the booster, and you will lose fluid.
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread "as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do! My drivers: 1987 190D 2.5Turbo 1987 190D 2.5Turbo 1987 190D 2.5-5SPEED!!! 1987 300TD 1987 300TD 1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere! |
#3
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Quote:
Thanks for the quick reply.
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John 1986 300sdl 1985 380se |
#4
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What he said.
If it is sinking, step down harder. It often will stop sinking if it is a bad master cylinder. Sometimes a brake job will cause this failure, some feel that it is due to pumping the brake pedal further than the normal travel to bleed, which can damage the seal as it crosses a wear-ridge in the bore. Yep, new MC time.
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Gone to the dark side - Jeff |
#5
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most MC"S come with a bench bleeder kit.
for some reason, the MB MC's don't. I think if you put a rag under your mc, install it, with the lines loose, then fill the reservoirs, they will bleed themselves. when clear fluid weeps out the line nuts, tighten them, then proceed to bleed the brakes at each wheel normally.
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread "as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do! My drivers: 1987 190D 2.5Turbo 1987 190D 2.5Turbo 1987 190D 2.5-5SPEED!!! 1987 300TD 1987 300TD 1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere! |
#6
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Quote:
My old brake hoses were almost swollen shut. Thanks for the input and advice!
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John 1986 300sdl 1985 380se |
#7
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Quote:
You do need to change out your BF regularly though, if you havent done that in a while, best do the change.
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Grumpy Old Diesel Owners Club group I no longer question authority, I annoy authority. More effect, less effort.... 1967 230-6 auto parts car. rust bucket. 1980 300D now parts car 800k miles 1984 300D 500k miles 1987 250td 160k miles English import 2001 jeep turbo diesel 130k miles 1998 jeep tdi ~ followed me home. Needs a turbo. 1968 Ford F750 truck. 6-354 diesel conversion. Other toys ~J.D.,Cat & GM ~ mainly earth moving |
#8
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1.Power brake booster seal is bad.
2. Napa parts sells a metric bleeding kit ( reusable ) .. about $10 when I got one last. 3. Do not try to install a MC which calls for bench bleeding without bench bleeding. I have that that in my younger days and the work taking it back out to do it correctly is totally wasted energy.
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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/showthread.php?threadid=10414 http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/showthread.php?t=156207&highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
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