Questions about troubleshooting the brake system on 82 300d
 

edit: all fixed


My reservoir is so grungy I didnt know it had two parts! And that you have to overfill the rear part to fill the front section. Read a few dozen threads and a few diy guides before coming across that fact.

[stupidity below]

While I thought I'd checked the brake fluid level less than a thousand miles ago, I was very surprised the other day when the pedal was extremely soft right as I pulled out of the drive way. At first the old stain line on the reservoir fooled me but then I checked the level by opening the cap and it was empty. :rolleyes:

So I started the brake fluid changing process with a hand-held Mityvac at the rear wheels and a quart of DOT 4 fluid. I was able to pull about 6-10 ounces of fluid (cant remember exactly) out but it no more came out. Even after greasing the threads on the 9mm nipples (which have alot of play) and replacing the flexible hoses at both rear wheels, I can't seem to pull new liquid through, only lots of air. I can hear it gurgling somewhere in the hard lines when I pump up the Mityvac before opening the valve. The reservoir level mightve changed after those first ounces were pulled but I can't get any more to come through, only bubbles.

The pedal continues to go straight to the floor and no amount of pedal-pumping changes the reservoir level. Even if one of the back 9mm nipples is cracked open. Shouldn't it push something into those lines?

I would assume the hard brake fluid lines are the next suspect, but I can't tell where it goes much after the rear T connection to inspect it visually. I suppose I could disconnect it up there to use the Mityvac and see if that isolates the issue past the split, even though I don't see any visual evidence of a leak.

Could it be something to do with the master cylinder? No visual evidence of leaks up there either.

I'm about to throw in the towel and let a good local mb mechanic to look at this.

If you didn't see any obvious leaks at the soft rubber lines (when I had a leaky rear line it was getting everything back there pretty wet), or out of the master cylinder in the engine compartment, you might want to pull the master cylinder off of the brake booster to see if fluid is in there. This is typically where "invisible" brake leaks are going.

I think I remember reading about that, that the booster can end up full of fluid. Sounds expensive.

I did just visually check the entire hard line running to the front and there was no obvious seepage or damage.

I'll go take a look at pulling the master cylinder. :X


I am still wondering however, where is the air coming from that I'm pulling? somewhere up around the master cylinder and booster?

The 9mm nipple(s), is this the bleeder valve on the caliper?

They should not have any play. Decent chance this is where you're getting air from. Or be greased, for that matter, but the seal happens at the end, no fluid gets to the threads. See red arrows, even looks as some come with thread locker.

As mentioned, you may be pumping fluid into the booster.

If you pull the master cylinder, have a new "O" ring on hand.

Glad it was an easy fix! :)