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Old 02-07-2005, 02:27 PM
Posts: n/a
What an idiot I am!!!

Upon an attempt to flush my brake system and install new synthetic brake fluid back in late Nov, I had come to the conclusion that my master brake cylinder was shot because I could only get pressure in the second stage after my first try (brake pedal almost to the floor) and with the weather closing in, I decided to let it go until the weather broke. In the meanwhile, I had ordered a new master cylinder, about $100. I installed the new unit the other day only to find out I still had no pressure in the first stage

I followed every direction I knew, and advise from fellow members here, so what the hell am I doing wrong, I mumbled to myself! I read the 124 bible again, nothing I had been doing was wrong. I filled the reservoir and connected up my mity-vac to the right rear wheel and pumped away....nothing. I disconnected the hydraulic tube from the master cylinder and put it in a jar filled with fluid and then pumped some more and then I started to get the fluid running out the right rear.

I remembered several people here mentioning not to over look the rear portion of the reservoir and to make sure it fills too. (there is a dam between front and rear of the reservoir) because it had been empty it appeared full.

Well, it wasn't!

The rear section of the reservoir dam represents what appears to be 1/16 of the total volume and it will not fill unless you rock the plastic reservoir back and foreth and watch for bubbles at the top as it fills in.

I wonder how many people have done the same thing? BB
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Old 02-07-2005, 03:10 PM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Well then the 124 is different than the 126. You still have the dam in the center, but, the fluid will easily flow over the top of the dam into the rear section.

It sounds like the clearance over the top of the dam is so small that the rear section can trap air while the fluid is trying to enter.

This would really make no sense.
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Old 02-07-2005, 03:30 PM
Join Date: Jul 2003
Location: Alberta
Posts: 266
I had the same problem with the brake fluid reservoir - the internal wall between the front and back halves is so tall that you have to fill the reservoir to the point of overflowing in order to get the fluid to pour over from the front half to the back half. Of course, I didn't know this when I started the job.

As I drained fluid out of the rear brake lines, I saw the fluid disappear from the back half of the reservoir but I couldn't seem to replenish it. Although I didn't know it at the time, the problem was made worse by the fact that I had the back of the car jacked up (as I was doing the rear brakes first) - this tilted the reservoir forward.

I finally gave up and decided to do the front brakes, since I knew I could refill the front half of the reservoir. Once I jacked the front of the car up, it tilted the reservoir enough that the back half started filling (and the job was easy from that point on).

This kind of thing happens to me all the time - most jobs (car-related or not) take 1/3 as much time to do the second time around, once you learn how things work...

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Old 02-08-2005, 08:57 AM
bhatt's Avatar
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Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421
In my W126, I ran into this problem the first time I flushed brake fluid.. kept getting air in the lines until I figured out the back reservoir section was empty!

I now fill the reservoir right up to the neck. That way there can be no air in the back section. When you're bleeding the brakes, keep it topped up to the point of overflowing and you'll be fine.

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