Tried this question buried in another thread; maybe more guys will see it here.

I'm trying to get a feel for how quickly the rag top hydraulic fluid might evaporate. In theory, it's slow (not zero) but I don't know how slow is slow.

Reason for wanting to know: let's say we encounter a 10 year old car for the first time. The fluid is low. Should we start looking for leaks, or would it be reasonable to assume evaporation, at least initially?

A good metric might be what owners who have had cars more than, say, 5 years have seen. Has anyone had a case where the fluid did not go down at all over such a period? If so, then evaporation is probably too slow to matter for our purposes.

Anyone?

TIA,

 

I think the question could be better stated. How about ...

If you've had a rag top long enough to observe how the fluid level holds over a period of years, could you please say what you observe.

Mine seems to go down at something like 1/16" per year, and I probably cycle the top ~ 50 times per year. I cannot locate a leak anywhere.

Thanks again,

 

Hydraulic oils will evaporate with the size of the molecules determining the rate. We had some top fluid loss but there wasn't any indication in front of the rear wheels, the usual place for leakage from the hydraulic jacks. Evaporation was suspected until I removed the rear seat to clean the seatbelts and saw oil under the seat in the area where the belts are anchored to the floor pan.

Possibly that is where your is going.

 

Thanks.

I'm trying to get a handle on whether the evaporation rate in this system is high enough to cause some measurable fluid loss without any leak, measured over a period of years.

 

I thought the convertable top hydraulic system was closed. There should not be any evaporation.

 

I'm pretty sure it must be vented. Otherwise we'd be working against pressure or vacuum in the reservoir as the fluid level goes up / down, which it does as the top goes down / up.

But if somebody knows otherwise, please give a yell.

 

Pentosin CHF11S is most probably the hydraulic fluid used and since it is synthetic the rate of evaporation, if there is any at all, is minuscule. If the loss is more than that it is leakage.

 

Right, Pentosin 11S it is.

There is a spec on the vapor pressure, which is a measure of evaporation rate. It is tiny, but I don't know how to guestimate whether over, say, 5 years it would be enough to see in the reservoir level.

That's why I'm asking what peope have observed, especially in a car with no known leaks.

 

Sorry, can't help with that as ours does have a very, very small leak which goes under the rear seat.

 

Seems like an easy experiment that would take about 1 year... Put some hydraulic fluid in a jar with a surface area about the same as the tank in the trunk, cover it with a piece of cloth to keep junk out of it, then leave it on the shelf in the garage for 1 year. See how much the level changed.
:-)

 

My top started going up and down more slowly and after reading tons of info here I thought I'd dig into the problem before it became worse. I found that my fluid level was very low. Mine's a 2002 XK8 that I have owned for two summers now. I searched everywhere for a leak and found nothing. I topped off the fluid and it seems to be working perfectly now. I'm holding my breath because I'm not sure where that fluid might have gone. Maybe I will try the evaporation test.

One other point. Gus has an amazing set of pictures that facilitated my project. My set up is different and the tank for the fluid was much more difficult to get to. It seems that the mounting bracket for my car is taller. It has a large space between the CD changer and an electric box, the purpose of which I am unaware. I had to remove the inner bracket that holds the CD changer in order to remove a nut behind the larger bracket that holds the hydraulic pump in order to refill the reservoir. It was difficult to remove even after all of the bolts were removed. I had to "kick" the bottom out being very careful not to break any of the pump electronics or crimp any hoses.

 

In my humble opinion there is no evaporation in the closed system. Look under the car at the front of the rear wheels for a 'wet' spot. The upper ram seals are a common leak point. The fluid shows up on the floor pan or under the rear seat.

 

I can detect no change in the fluid level since '04..

 

50 times per year??? I probably do that in a month. Heck, it went up and down 4 times today. That's 4 ups and 4 downs. I'm doing it even more now that I have the nifty remote control top.

 

Tom,

But is the system closed? If it is, then as the top is raised and lowered, and the fluid level drops and rises in the reservoir, we'll be creating pressure and/or vacuum there, which the pump will have to deal with.

What do you think about that, my friend?

 

I doubt that the system can be sealed, there must be a vent of some sort although this will likely be small.
I also doubt that there will be any appreciable evaporation of the hydraulic fluid, I'd think any losses will be due to seepage from the various seals.

 

Please read the following TSB about UNIVIS fluid.

That is why the CHF11S was used instead.

bob gauff

 

New to forum but I installed relief valve this past weekend. 1999 xk8 with fluid about a quarter inch low befor the install. I filled it up. Monday and Tuesday I layed down newspaper under rear wheels to check for leaks. I always put the top up and down at the same spot in the driveway So far no leaks. I will check for leaks under rear seat. Maybe fluid is not always full to begin with at the factory? mick99

 

Bought my '98 XK8 in Dec. '06. Original owner '98 to '05 nothing in Jag files anything done to top or hydraulics. Second owner '05 to '06 I would be surprised if the idiot did any maintenance on the car much less check hydraulic fluid for top. When I got it the fluid level was perfect between the hash marks. Changed fluid 2010 level was still perfect. Guessing no leak (knock on wood) or evaporation. Jack

 

When you changed the fluid in 2010 was the fluid yellow or green.

Univis = yellow
CHF11S = green

We changed the fluid all the time at the dealer both in warranty and customer pay if the top was slow.

Here in Texas we have high summer temps so the fluids get changed regularly.

bob gauff