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#1
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Hard Brakes, Booster Bad?
This is on the 79 300 TD.
I haven't driven it much at all, but the brakes are hard and it is difficult to stop. I bought it in November and parked it in the garage with the intent of only driving it in the summer months. I've been working my way through some of the issues. It had hard brakes when I got it. This is what I've found so far. -23.5" of vacuum on the nipple at the main line (only one nipple on my model) with the engine running leaving the booster line hooked up. -Booster holds at least some vacuum because after I shut off the engine and unscrew the main line at the booster there is a rush of air as it equalizes. -I definitely have at least one leak in other systems (maybe two). Somewhere in the climate control system (probably pods) is leaking. -I think I fixed the shut off leak. -I drove the car with the main line nipple plugged so the only thing hooked up to the vacuum was the brake booster. Still had hard brakes. -Thought it was the check valve so I replaced it (closest to the booster). Seemingly no change. -Door locks move slowly while engine is running and after shut off. The lines hold vacuum with the MityVac from under the hood. -Tried to depress the brake pedal and shut off the engine (per booster checking suggestions from here- http://www.autoshop101.com/forms/brake05.pdf ) and the engine would not shut off until I took my foot OFF the brake. Then about 5 seconds later it would shut off. Did not matter if I held the brake for 10 seconds or two minutes, same result. -Depressed the brake pedal 3 times after the engine was off and it seemed to go to the same position. I guess after doing some research on here, I need to recheck the booster to see if it is leaking. I tried but gave up after maybe 15-20 pumps and no vacuum was created. Anything jumping out at you that I am missing? Thanks!
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2009 ML320 Bluetec 1985 300CD 1981 300TD Past Mercedes 1979 300TD 1982 300TD 2000 E320 4Matic Wagon 1998 E430 1984 300SD 1980 300SD |
#2
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If you have 23.5" with the booster connected then it's not likely a leaky booster. I've had issues with the pump pulling enough vac but it couldn't keep up with the volume needed after I pressed the brakes a couple times. Your pump may need a rebuild.
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Andrew '04 Jetta TDI Wagon '82 300TD ~ Winnie ~ Sold '77 300D ~ Sold
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#3
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Take a vacuum reading on the mainline with your foot on the brake. I'm betting it will drop significantly. I suspect a bad booster that's leaking once you step on the brakes.
It takes a lot of pumps on a mity vac to get vacuum in the booster. I'd say 50 or 60 at least.
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13 |
#4
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Seemingly it happens the first time I press the pedal. Although I do have a rebuild kit (in a parts box from the PO). I'd like to rule everything else out first.
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2009 ML320 Bluetec 1985 300CD 1981 300TD Past Mercedes 1979 300TD 1982 300TD 2000 E320 4Matic Wagon 1998 E430 1984 300SD 1980 300SD |
#5
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Quote:
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2009 ML320 Bluetec 1985 300CD 1981 300TD Past Mercedes 1979 300TD 1982 300TD 2000 E320 4Matic Wagon 1998 E430 1984 300SD 1980 300SD |
#6
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Update
I left the MityVac hooked to the main line at the only nipple overnight. When I left last night it was holding at 16" vacuum (It was 23 but then I unhooked the brake booster momentarily to check if there was vacuum there). This morning it was at 0.
So then I put the gauge on the windshield so I could see it with the car running from inside the car. I started the car. I let vacuum build and pressed the brake pedal. It immediately dropped to zero. I decided I would let it run for awhile. I let it run maybe 3 minutes and checked again. The gauge again immediately dropped to zero when the brake pedal was depressed. I assume this is not normal? I also assume this points to a leaking booster? I also checked for vacuum with the engine off. I hooked the MityVac directly up to the booster and pumped maybe 125-150 times and got the vacuum up to 10". It held for at least a minute. I couldn't keep the seal with out hold the line with my left hand so I couldn't test longer unless I wanted to stand there holding the seal longer.
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2009 ML320 Bluetec 1985 300CD 1981 300TD Past Mercedes 1979 300TD 1982 300TD 2000 E320 4Matic Wagon 1998 E430 1984 300SD 1980 300SD |
#7
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The booster doesn't use vacuum until you press the pedal. Then if there is a leak it will drain your vacuum supply and you should hear it also. Your test indeed confirms a bad booster and it isn't hard to replace. Don't unhook the brake lines. Just unbolt the master and gently bend the lines out of the way.
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1977 300D Lost coolant while someone else was driving 1983 300D Can't run without oil 1985 300SD (gone but not forgotten) 1990 300TE 4matic Sold 1991 Yamaha Venture 1975 Kawsaki 250 triple 1974 Honda 200CL 1951 8N Ford 2008 Wildfire 650C |
#8
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Quote:
I probably will remove the master cylinder as I want to do a flush anyways and will probably replace the hoses at a minimum (I'll check the pads/rotors when I'm in there). I have a good booster from a 1980 300SD I parted a few years ago. It is physically different. Any chance this part would work? (I doubt it, but figured I'd ask)
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2009 ML320 Bluetec 1985 300CD 1981 300TD Past Mercedes 1979 300TD 1982 300TD 2000 E320 4Matic Wagon 1998 E430 1984 300SD 1980 300SD |
#9
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To change out a brake booster, can anyone confirm that the master cylinder lines can be left intact? Thanks! I need to pull mine.
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#10
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There is an O-ring between the Master Cylinder and the Brake Booster that also seals in Vacuum.
You might want to check that and also to see if there is Brake Fluid pooled in the Brake Booster. Some people have loosened the Master Cyliner retaining Nuts and shoved the Master Cylinder Forward some to do some inspecting. I think shoving the Master Cylinder backwards and forward bends the Tubings a little.
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84 300D, 82 Volvo 244Gl Diesel |
#11
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Brake booster replacement, no master cylinder removal
I was optimistic the booster could be replaced without removing the master cylinder after reading some other posts--couldn't find the thread I originally followed, nor the one with the excellent booster diagnostic procedure--will edit if I can find them.
Took less than an hour--brakes working well now. There was one little thing that bothered me--is the fresh air source for the booster located inside the cabin in a little hole in the grey boot? I still hear a bit of a hiss when I'm braking, but hard pedal has gone away with the replacement (used) booster; thought the hiss might always be audible if air source is in the cabin.
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1980 300D (non-turbo, 421k miles) 1984 300TD (turbo, 305k miles) 1985 Vanagon (170k miles) Last edited by nscarr; 11-12-2012 at 10:56 AM. Reason: file didn't attach |
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