I just replaced the vacuum modulator on my wagon the other day and noticed that it's clearly leaking fluid (pretty bad) from the modulator, as if it's not secured on properly. The thing though is that the two 5mm hex bolts are on tight. It seems that the leak is coming from the bottom side of the modulator - maybe it's just not on completely and a bit loose/crooked on the bottom since the mounting bracket bolts on the top end anyway.

I have the small o-ring that goes on the inside to seal the inside of the housing. I'm assuming that's the only seal for the modulator (I don't recall any other one on the previous modulator.

I've tried to put the modulator on twice, both times they leak bad. I put the metal bracket on the modulator that holds it in place, slide the unit on, but it still has about 1/4 - 1/8 inch of a gap between the mounting cover/bracket and the bolt holes. I'm able to screw it on, and I am assuming that as the bolts go on tighter, that it slides the modulator on fully snug, but I'm guessing maybe it somehow needs to be on snug before the cover gets put on and bolted in place. I've tried using a wrench, crowbar, etc to push it in further, but it's really hard and I haven't had much luck with that. So I resorted to using the bracket and bolts to push it on all the way. Maybe by using the bracket/bolts to push the modulator on it only does so on the top end and the bottom end still is a bit loose/crooked.

I'll try it a third time and lube the interior of the modulator and housing, to see if it'll slide on completely and THEN put the bracket/cover on after it's fully in place - but are there any other tricks to make sure it's on correctly? There's just the one o-ring that goes on the inside of the modulator and housing/socket, right? Does the bracket/cover go on the modulator before you slide the modulator in, or after?

Any suggestions how to get it to not leak would be helpful.

Thanks,
Tomas

For whatever reason I found the new modulator o-ring was cut and probably not creating a proper seal. I took the advice of another thread and went to harbor freight to grab an o-ring kit, but I picked up the metric kit and I think those are all way too thick. I've tried to fit it on and no matter what, either the modulator wouldn't attach or it would still leak.

At this point I just gave up and stole a modulator + o-ring from another car (that has worse problems). It works completely fine on the first try (after several hours of trial and error with other o-rings). No leaking.

I could not put the old o-ring on from the previous modulator since it was lost in the process of putting the new one on. I'm normally pretty militant about keeping old parts but I decided not to this time =(

Instead of fiddling with o-rings that may not even fit, I have some time now (and a working vehicle to get to work) so I can take my time and order some of the OEM o-rings - at least 2.

One thing I noticed about this donor modulator+o-ring is the unit is an older green style modulator (with the black rubber cap) where the o-ring fits in the middle of the modulator, in a recessed groove rather than slid on the end like with the newer green modulators (with the dials). The newer modulator+o-ring seems to have the o-ring go on closer to the end and so I'm not sure if that is somehow causing a seal problem.

Generally speaking I am telling myself how much I like the W126 (300SD) better than the W123 (300TD). I don't even need to lower the transmission crossmember in order to get the modulator in and out on the W126. Took me 2 minutes, compared to the 10 minutes (x 5 attempts) on the W123 wagon. =(