Had the master cylinder replaced about 6 months/2,500km ago with brakes working fine until yesterday. Suddenly the pedal goes to the floor with braking power non-existant until the last half inch or so. Car will stop without problem from moderate speed (25 - 35 mph) but I really don't want to tempt fate with higher speeds. From the feel of it, I doubt there is enough grab to lock the wheels and again I don't wish to tempt fate trying, and then finding additional damage from doing so. Brake fluid reservoir is still pretty much full. Pumping the pedal results in no discernable change in feel or reservoir level.

Any ideas on what i should be looking for to zero in on the problem would be appreciated.

TIA

Those reservoirs can be deceiving. The front part is the rear brakes and the rear is for the front. I suspect that the rear part of the reservoir is out of fluid. Check for a problem at the front brakes and check for leaking seals at the bottom of the reservoir. Just my observations after 30 years as an MB mechanic. Take it or leave it.

peter

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Thanks Peter. I'll take a look this evening and report back.

Patrick

Have inspected brake fluid reservoir (no front/rear compartments) and it is full of fluid and there are no signs of leakage visible.

With engine idling I did pump the pedal rapidly and felt a build up of pedal feel before the engine revs dropped to the point of stalling the car. Throttling up to just above 2,000 rpm, I was unable to get any sort of pedal feel from pumping the brake pedal, however when i dropped back to idle, I was able to "tease" the revs down to just over the stalling point by pumping the brake pedal, at which point there was some resistance felt.

Any other troubleshooting tips would be gratefully accepted.

First thing you should do is bleed the brakes! Air may have gotten in there! Check all lines for leaks as well! Then last the vacuum line to the brake booster!

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

Despite being very new, it still smells like the Master.

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1986 300E 5-Speed 240k mi.

Found what looks to be a broken plastic vacuum fitting near the temperature sensor for the fan clutch. red platic with two pipes coming out (1¼ now ) with rubber connections leading to other plastic tubes. One of the tubes busted near the base. Any ideas on what the name of this part is so I can order a replacement? One pipe is vertical and the other not quite vertical, but angled slightly. ( _||_//_ )

Part could have been/probably was damaged when I replaced the sensor for the electromagnetic clutch which is on the same assembly about 2 inches or less away from it.

Wondering if this could be affecting the brakes since they began acting up shortly after the sensor was replaced.

TIA as always.

Patrick

I would be inclined to think of a bad booster if you can stall the engine by pumping the brakes, it shouldn't use that much vac. Check the line (probably separate).

The broken fitting won't help, but wouldn't cause your problem.

You need to bleed the brakes and check the brake line hoses, it's possible one is bad.

Lack of vac at the booster makes the pedal hard, not sink, so I also would consider the fact that the "new' master may be bad, it acts just like a bad master cylinder!.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thermo Vacuum Valve! It actually controls the release of fuel vapors from the Charcoal Cannister and connects the intake manifold vacuum to the fuel cannister area on the fender to open the valve and allow fuel capors back into the intake! Opens at 50 degrees. 22 buck part. and I broke mine doing the same thing! You can get way with just routing the line from the intake side to the Y adapter so that your vacuum advance will still function! To see if that is the problem. I re-did all my vacuum connection a month ago and now things work a lot better. Cracked rubber vacuum connections can effect everything from braking, heater control, advance, and shifting of the auto tranny. One leak and the vacuum level drops in the intake manifold ever so slightly, a few and you get more drop!

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

Discovery #1 - when a tech with many years experience says one thing and your eyes see otherwise, check your eyes or look at it differently until you see what he's saying. (thanks peter, rear resevoir drained)

once refilled, i depressed the pedal and hear a spritzing sound... sure enough, there's a leak near the rear left (driver side) wheel. diagram to follow....

http://charteq.corp.eol.ca/20040829hc5.gif

Leak Diagram

Is this a PITA job or one that could be considered by a relative DIY novice?

many thanks to all who have replied.

Patrick

Patrick,
You need to determine if it is the hose or caliper. Change the part according to the manual, fill the rear reservoir, open the bleed screw and gravity bleed til fluid runs with no air. Close the bleed screw and you are done.

Peter

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Auto Zentral Ltd.

Hi Peter. Thanks for the post.

Would the procedure be similar for a w126 and if so is it on the CD?
(I have the w126 cd but not the w201)

I suspect I'll have to replace the brake line from the bracket forward.

it's quite rusty where the metal tube transitions into hose that goes to the caliper.

Best regards,

Patrick

Patrick,
I'm a little confused. You say left rear wheel has leak. Traditionally the rear of Master Cyl. goes to front brakes. Perhaps ABS changes that. If you meant left front, the metal line should be easy to change. Left rear is a different ball game. I would try soaking the flare connection for a day or so with a GOOD rust penetrant ( NOT WD-39 and a half). Then use a tubing wrench to try to loosen the fitting. If you don't have any luck, you can take it to a shop that can cut the end off and install a new connector with a flaring tool. Changing hoses and calipers is basically the same for the cars you mentioned.

Peter

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Auto Zentral Ltd.

I will add the following:

If you have access to compressed air, blow out the connection where the nut goes over the line. Soak down with PB Blaster, etc (as stated, WD-40 is useless for this), allow to stand a day or two. Blow out with compressed air again (use eye protection!!).

If you have access, get two hammers in there. Place one on the nut, brace it, and give the fitting a good thump with the other, dead square. You must be carefull not to smash the nut flats, but you want to give it a good whack. Blow out again (the idea is to get all the road dirt out of the space between the nut at line, so it won't jam up when you turn the nut), soak down again with PB Blaster, Kroil, etc. Using ONLY line wrenches (the kind with five large flats that fit over the line), you should be able to crack it loose without damage. The normal MB size is 11mm, not in the usual sets, you will have to get it separately.

Don't use vise grips unless you are planning to junk the line anyway, as they always flatten the fittings, making them usually impossible to reassemble leak free.

Note that a bad hose can squirt fluid straight back, the line may be OK.

The good news on all of this is that I have only had one fitting give me much trouble -- a badly corroded one on the 72 220D. Al the rest have come loose nicely, including the four hoses on the 280SE.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Sounds like good advice. Looking at the locations of everyone on this thread, I'm the last person to be giving advice on loosening rusty fasteners. We don't get much call to practice those techniques out here in California.

Peter

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Auto Zentral Ltd.