W124 soft brake pedal - Assist failure

[jonbobshinigin]

Ok...not sure how to describe this so here is my best shot:

Brakes have been fine. Fully functional ABS, brakes and rotors are 15-20k old. Brake fluid was changed and bled then, and I think one other time between now and then. I get into my car one morning to drive off and go to stop the first time and it is noticeably soft and did not work till maybe the last 1/4 of pedal distance. The day prior is was working. Next morning BAM! It basically feels like I have no assistance from booster.

I have not done any recent work on the car and vacuum system, which I addressed a long time ago, checks out ok.

When I press the pedal, I can hear the swoosh. If car is off and I depress the pedal, they will tighten up. But if the car is turned on, I get nothing. I also tested it today and was able to lock the brakes up.

I have read that there is an oring between where the master cylinder connects to booster. It is a square oring but can be replaced with a round one and that it resolved a lot of issues. I pulled the master cylinder away from the booster and identified the oring which does not appear to make a great seal onto the booster surface.

What do you think? Keep in mind the was not progressive. One night good, next morning bad. It may have gotten worse the one day I have driven it.

[vstech]

this sounds like you have no fluid in the master cylinder chamber.
check there first.

[jonbobshinigin]

There is fluid in the reservoir, is that the same thing?

[vstech]

yes. but you know there are two sections to it right? hard to see. make sure both are FULL.

[babymog]

Able to lock the brakes? Sounds like you have multiple failures.

[jonbobshinigin]

Ok, so I am going to check for fluid in both chamber of Brake Fluid Reservoir correct?

[vstech]

that's a good place to start. correct.

[jonbobshinigin]

The reservoir is right right in the middle of Max and Min. It appears that both chambers are full but I didn't really see two distinct chamber when pulling the cap and looking in.

[vstech]

you won't see chambers from the cap, but if you look carefully at the passengers side, you will notice the parting line between the two.
if yours are full, I'd suspect a bad MC, your pedal should not go to the floor if you have good fluid supply.

[jonbobshinigin]

Is this a rebuild item or should I purchase a new one?

[jonbobshinigin]

Well...it appears me rear driver's side caliper is leaking...I a, researching rebuilding it vs getting a new unit.

[vstech]

a rebuilt one is quicker and easier, but you should be able to rebuild it yourself if you can spare the time.
or if it will wait until you get here, I have a few sets sitting on parts cars... including the 240 rustbucket! but since you are talking about your TD, I don't know for sure. the 93D may have parts that would work, but you should check and see if they well interchange.

[vstech]

nope, different part numbers. a rebuild is not available on the allparts express, only new... nearly 300... ouch. a seal kit is available though...

[jonbobshinigin]

Ok, I changed the rear hoses and when trying to bleed, I get nothing. I had someone help me. With bleeders close: pump, pump, hold, bleed, tighten, repeat. A mechanic friend helped me who knows more than me. Anyhow, they did not bleed or push hardly any air out even. Then we tried to let it gravity bleed to no avail. What do these symptoms sound like? Master Cylinder?

[jonbobshinigin]

I retried the bleeding process after filling the reservoir to the brim and finally got them to bleed and flow a solid stream. The pedal is still very soft and goes to the floor. Master Cylinder?