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Hydro strut weep - normal?

Started by daantjie, 08 November 2021, 11:45 AM


Hi guys

Calling the 6.9'ers here...Brain fart time...I read a bit on the forum here about what some folks used to describe as "normal" external weepage from the hydro struts.  I profess that my knowledge of hydraulic cylinders is limited in general, though I know my way around the HPF system.  I am not sure how these "wiper" seals are supposed to work?  I believe the seal at the bottom of the struts is the "seal of last resort", so if it wears you will see fluid weep into the cups and out the drain holes on the front struts for example.

So is some weep here normal, or should they seal tight?  Also then if weepage is normal, I am guessing a drip over a few weeks is OK, whereas a puddle forming is obviously no bueno?

1977 450 SEL 6.9 - Astralsilber


no, the struts should not weep.
Remember that the hydro room behind what you describe as "seal of last resort" is connected to the transparent return line. And this lines is connected to the return lines of the other 3 struts and the return line of the axle valves.
So this "room" might get oil from an axle valve when it tries to lower the car. And it gets oil from the internals of the strut in case there is a leak.
In case the piston rod is wet your strut is crying for a rebuild.
But do not worry if you see oil in the transparent return line - this might come from an axle valve.


OK great thanks Thomas for the info, this is good.  Yes for sure I am clear of the "controlled" leak by way of the clear plastic "D" return lines back to the tank.
1977 450 SEL 6.9 - Astralsilber


The rebuild is really just replacing the 2 sets of plastic seals and all the orings.  The plastic seals get scratches in them and thus leaking occurs.  The main hydraulic seal is the upper "piston" ring.  I am not aware of anyone that has ever replaced that one.  I dont even know how you could get one if you wanted.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,


There are effectively 2 seals at the bottom.  Both are static...the ram goes up and down only thru one of them.. The lower cap has the "2nd" seal. If there is an internal leak, it is becasue the "!st" seal has failed and a tiny little bleed hole in the cyl allows it to escape and into the tube that takes it back to the resevoir.
I am currently rebuilding some struts and it is quite entertaining!

The top seal is indeed a piston ring......there is also a bronze teflon seal. I have both of these as the first set of struts I encountered were rusted and these rings were frozen. I also had to make the cylinders as they were U/s.
"No project too bold !"...hahaa!

The front struts are the most vulnerable to wear....the protective cup is more trouble than its worth. Dirt collects in it and over time builds up and compresses such that the stroke of the ram is reduced. This gives rise to the rubber bump stop chopping out. Over time the ride becomes impossible.
Oddly enuff, it's all the outside stuff that wears out.  Ball joints mainly because the rubber boots are never replaced...bump stops because the pots are never cleaned.....the pots themselves becase they rust out having collected the mud..and the rear gaiters and ball joints coz people just let them rot away.  These were all scheduled service items yet no one bothered. The struts themselves are good for 50 years.

bottom line: don't worry about a little weep.


Indeed Randy I have seen first hand what can happen on the ball joint boots on the rear for example.  The mount for the strut is quite recessed into the control/swing arm, so this is great place for dirt to accumulate.  There is a drain hole in there, but of course over time it clogs and no one bothers to check it, so you get this "sandpaper" effect on the dust boot which of course eats it away and then it's overs.  Also on the rear the dust gaiter fails as well, and then you get ingress which will also make it's way into the piston/shaft and then the same degradation happens to the seals.
1977 450 SEL 6.9 - Astralsilber