I have a new-to-me 1984 300TD that I am trying to sort out. The brake pedal is losing firmness and I have to pump it once or twice to get it to firm up temporarily. I recently bled the system to see if that would help but without any effect, so I think the master cylinder is on its way out. However this model has ABS and the ABS light is always on.

Question: I'm unfamiliar with ABS so i'm wondering if ABS problems might cause soft brake pedal problems?

Bonus question: there are two hard brake lines on the passenger side of the engine bay of this car, near the passenger side wheel well and going toward the rear of the car. In this particular car they are disconnected and I'm not sure what they were used for? Any ideas?

I doubt that ABS could cause the mushiness. -can not confirm on these early systems.


First thing I would do is check for brake fluid leaks and worn out rubber hoses.

then, check all the rotors for runout / abnormal wear patterns

ensure that all pads were wearing evenly.

ensure that ALL caliper pistons are moving freely (ie... not frozen or sluggish).



...if all of that checks out, pull the master from the brake booster and check for fluid in there.


if no fluid inside, bench bleed the master (<-- skip this if you have witnessed the deterioration of braking ability) or replace the master.

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could also be the rubber seal between the booster and mbc. Mine was a little mushy and replacing the seal fixed it.

Those lines may be for the SLS system for the rear axle. Is that system still intact? If someone has removed the SLS from your wagon, they probably left the supply / return lines behind.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

@Maxbumpo - right! Not sure why I didn't think of that. This 300td does have the SLS deleted.

@ambush276 - thanks, changing that seal is probably worth a try. Is it possible to change the seal without removing the any of the brake hard lines and having to bleed the system?

If that doesn't work I suspect i'll just plan to rebuild the entire braking system. The ABS system makes me a little nervous since I've heard it can make bleeding the system more difficult.

If the ABS light is on check speed sensors, very common problem.

On earlier 124's they were on each of the front wheels plus one diff mounted, they do lose magnetic strength over time or reluctor ring could be damaged/rusty (either integral or heat fit).

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David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

I'd start by doing a normal brake bleed, after checking the calipers for leaks.

2x on a bad sensor causing the ABS light.

-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.

More likely pedal firmness is caused by a bad master cylinder or a system in need of a flush with fresh fluid. I myself have never been successful in bleeding and ABS system on a Mercedes... I would take it to a shop and pay for them to pressure bleed it.

Also, does your tach currently work? If not, a dead tach and an ABS light on can point to a OVP relay that needs to be replaced or have its fuse replaced.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

This requires pictures and could be a serious issue left by a previous owner that didn't know what the hell they were doing...

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

Before I started this thread I bled the system really well without any improvement. This last weekend I replaced the master cylinder with a new unit. I bench bled the MC and rebled the entire system with my motiv style pressure bleeder. The new MC did the trick, brakes feel great now.