1992 400E Brake Pedal Mushy after vaccum bleed
 

Having searched and read the mushy pedal threads here is the problem I am
encountering and was wondering if
anyone had suggestions...

I just replaced front rotors, brake pads and calipers on my 1992 400E with 110K miles. (The two rear calipers, rotors and brake pads were replaced last year.)

Then bled all four calipers in sequence (rear right, rear left, front right, front left) using the motive power bleeder. Fluid ran clear without any air bubbles at all four corners...I bled out about 1.5 pints at each caliper. Car was level on four jack stands.

I used Valvoline Synpower DOT 4 brake fluid.

The problem is that the brake Pedal remains mushy and I am unable to
figure out what is causing this.

Any ideas on what else I should look
at.

This is just silly, but hey, it happened to me...
And no one else offered any ideas...

The reservoir is divided into 2 chambers, front chamber for the front brakes, rear chamber for the rear brakes. I found that the rear chamber only gets filled when the front chamber overflows into it, which surprisingly happens just at or perhaps a bit above the 'full' line.

I was running the rear chamber dry, and it took me some head scratchin to figure this out, since the front read full. If you clean up the outside of the reservoir, you can see where the rear chamber level is at.

Your reservoir may differ from mine, since you have a V8, and I have a manual (with clutch supply).

Checked that!
 

Csnow,

Checked that....I can see fluid up to
max. all around the reservoir.

I also noticed that when I maintain constant
pedal pressure it does not fall away (I was
thinking about master cylinder going south
perhaps?) If I apply increasing pressure
on the pedal it will fall...feels like compressed
air bubbles but I just cannot find where they
are after the brake bleed .

Thanks again!

Was worth a try...

Anyways, it seems unlikely that your MC would just coincedentally up and fail after a brake fluid flush. A pressure bleed like you did should be easy on the MC besides.

No new ideas. Hope you figure it out....

Re: 1992 400E Brake Pedal Mushy after vaccum bleed
 

Hmm, I just bought the Motive power bleeder and had planned to flush the brake lines on my ML430. Now you got me worried that I may screw things up. Was your brake mushy before the power bleed? Thanks. -Norm

when you vacuum bleed, it is possible for air to get into the system through the bleed screw. don't ask me how this is possible, I just know that it is, but it never happened to me when i vacuum bled. A lot of people will tell you that pressure bleeding is preferable for that reason. I got a motive pressure bleeder a while back and haven't vacuum bled since. I would try to vacuum bleed the brakes again and see if it helps.

why not try manual old fashioned 'two person' bleed and see if that helps the situation. after all it's worth a try.
and here's an article that might help with the situation though unlikely given your pedal was fine before the bleeding.

here's for a faulty M/C diagnosis.
http://www.babcox.com/editorial/bf/bf120312.htm

and here's one to troubleshoot a mushy pedal.
http://www.babcox.com/editorial/bf/bf60133.htm

good luck and please do post the progress. many would find the troubleshooting helpful i'm sure.

nbml430,

Pedal was fine before the
motive power bleeding. I used the motive unit successfully last year when I replaced the rear calipers, rotors & pads. I am sure something odd occurred during my front end brake job last weekend....just trying to get to the bottom of it.

Schumi,

I am beginning to think that is what
happened....just not sure yet how it
occurred yet...but you are right I will
probably re-bleed the brakes next weekend to see if I can flush this
air out.

Benz300,


Good articles! Thanks for the pointers.
I think between these articles & the
advice posted in this thread bleeding
all 4 calipers again is the only logical next step. I just think I will utilize the motive unit again and if that still does not do the trick then revert to the two person method.

And yes, I will definitely update as I go along.

Two additional questions: what is the maximum pressure can I pump the MB W124 brake system up to i.e. the motive unit maxes out at 30psi?

Is 25 psi pressure that I used too low?

you can also try buying the 'one person' bleeder screws. they have a check valve built in hence minimizing the entry of air back into the system through the screws while bleeding.
good luck.

Where to buy?
 

Benz300,

Where could I get those?

i saw them at pep boys, autozone, and advance autoparts. in the brake tools section.

I think he means these:

http://www.speedbleeder.com

They do work well, though they are expensive.

yup,
these are the ones I was referring to.

Thanks for the speedbleeder pointer!
 

This morning when I started her up...the
pedal seemed a lot less mushy! Somewhat
odd...hopefully will have time to re-bleed
this weekend. Will update this thread once
I get that done.

bleeding
 

FYI...

I use the power bleeder which pushes not vacuums.
- remove old fluid with turkey baster
- top to the rim with new fluid
-pump to 15psi
-bleed four corners
-remove excess with turkey baster until at max mark.
done...

I've done this twice without any problems. I'm thinking some air got into your systems when vaccum was pulled and the level was close to MIN mark maybe since it was at 30psi. I think I read somewhere that 15psi is max because of the ABS, don't remember where. I'm just shooting at the dark here. Try it again and ensure reservoir always above max OR do the 2-man job OR get a pressure bleeder.

Considering that the reservoir has 2 chambers, visually checking the level does not guarantee that both are always full with the vacuum method, but it does with the pressure method above.

Good luck,