Yesterday I changed front calipers on my 300SDL. I was bleeding with pressure bleeder two times and still ...the pedal is going down when I press the brake as if the brake can't build up pressure.
The brakes work but if I stay on the brake pedal goes down.
Anything I did wrong?
How good are chances that the master cylinder is shot?

Are the bleeder valves on the top or bottom of the caliper?

You may be ok on the brakes...and the Power booster be leaking...
There is a good thread in the archives.. use my name for search...

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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

just had this same problem; turned out my new rear calipers were in the wrong boxes, and I put the left on the right and vice versa, with the bleeders at the bottom. switched them to the proper sides, with the bleeder at the top, and no more troubles.

Also, if you let the reservoir run dry while you had the calipers off, you may need to also bleed the MC, OR, if your MC is really old and crusty and you pumped too far down while bleeding, you might have screwed up the seals inside.

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1976 Mercedes 240D, unknown mileage
1977 Mercedes 240D, 225k
1992 Dodge/Cummins 4WD, 284k
1990 Subaru Legacy wagon, 330k
1991 Subaru Legacy wagon, 225k

Top.

Searched and found your thread.
Well...the reason for a sinking pedal might be

1) air in system......well I did double bleeding
2) mastercylinder failed...probably because I hit the pedal too hard during bleeding?
3) brake diaphragm is not holding vacuum - diaphragm shot or..the rubber gasket?

But why should the diaphragm go bad all of a sudden?
I could have pushed the brake pedal harder than I should...at least this could have happened...

So how can I trouble shoot this?

A) I could try and see if booster diaphram holds vacuum over a period of time?
How is the correct test procedure?
B) I could try an bleed again...maybe somehow a lot of air entered the system....but I doubt it to be honest.

C) Is there a test for the master cylinder?

The reservoir must be FULL to bleed the rears. Like refills virtually every pump.

__________________
James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

Also, make sure the vacuum pump is developing sufficient vacuum. If not, the symptom may appear as a slight sinking pedal with hard grab near the bottom.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Earlier you mentioned pressure bleeding. Are you using external pressure such as from a Motive Power Bleeder or pressure through the pedal?

How many strokes of a MityVac does it take to evacuate a brake booster?

Sixto
87 300D

You most likely need a master cylinder. A booster failure would cause a hard to depress pedal, not a sinking one. Why were the calipers replaced?

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

http://www.peachparts.com/shopforum/showthread.php?t=250621&highlight=sinking+pedal

__________________
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

You described "bleeding with pressure"...can you elaborate? I suspect you still have air in the lines and have not bled correctly. I recommend using a motive bleeder. You don't have to touch the pedal with it, just fill it and open the bleeders until you get clear fluid.

You do realize you need to bleed all 4 wheels even though you only opened the front lines, right??? Start with the wheel farthest from the MC and work your way back.

You can run the risk of ruining the MC if you pump the pedal too far. That's why I like the motive system...no chance of that happening or of the MC reservoir running dry. It is one perfect system and virtually idiot-proof.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Hi Sixto.

I did use a power bleeder with 10-15 psi air pressure.
On top I hit the brake pedal several times because I had to get the brake pads tight to the disk.

I'll try and test the evacuate the brake booster and see if it holds vacuum ..and report back

nhdoc: I didn't know that I need to bleed the rear brakes also even if I just touch the front ones!

The only thing I might have done wrong is pressing the pedal too hard when I
tried to move the new brake pads close to the disk.
However I'll try and bleed again ...All four...and report back

The old calipers were still fine and brakes were actually good. I did change them and the flex lines because of age... car had 330 k miles...preventive measure.
Stupid to change a well running system.