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MG MGF Technical - Hydragas evacuation/ volume query

having successfully built & used my own modified bottle jack pump, the next step is of course to lower the car properly.
The knuckle pins are en route, but what is the best way to depressurise the system? I have seen pics of a Hydragas evacuation system on Dieter's site: any more details??
I suppose I could just get some connectors and run them into a bottle with the car on the ground, but there is then the problem of getting a jack under the now lowered car.
Axle stands? (more expense)


Also, what is the volume of fluid per pair of hydragas units?


ps the only mod. I made from Dieter/Carl's pump was to get a proper section of high pressure hydraulic hose for the schrader connecter as the original section bulged dramatically when it got hit by 400psi.
Charles Dundas

When I've fitted shorter knuckles the procedure has been to fit them, drop the car back onto the ground and then connect the pump to each valve in turn, let the fluid escape until the car sits well below the desired ride height, and then pump back up using Rob's Rule of Thirds until the right height is achieved. I believe full evacuation would only be necessary of you were dismantling the hydragas circuit, e.g. replacing a sphere.

I admit I used a Liquid Levers pump, but as far as I can see Dieter's version will work just as well.
Mike Hankin

Did you have your car on stands before evacuating the hydragas?
If not, how do you get axle stands under the car once lowered?
Charles Dundas

IIRC there's no need to touch the hydragas in order to fit the shorter pins, so it's two separate processes. The fluid release & re-pumping is done afterwards with the car sitting on the ground as normal; I was surprised at how easy it is to lift the car with only modest pump pressure. That's the beauty of hydraulics I guess 8-)

Axle stands would be advisable for the fitting of the pins for the usual safety reasons, but an MGR scissor jack hasn't failed on me yet...
Mike Hankin

Charles,

as for evacuation, Carl used a double acting pressed air cylinder.
http://www.mgfcar.de/pump/scuction.jpg

You only need it if air got into the system.
If you just release fluid when the car is on wheels, then it's important to put back the threaded covers to the schrader valves and close them sealed. So no air can get back into the system while you change the pins.

Drive the car on some wooden plints to get enough height for a hydraulik jack at the lowered car. Axle stands for security also.

Dieter

A cheap way of getting vacuum is an old refrigerator compressor. I've seen this used for vacuum forming, it works really well. Another projesct still under the bench waiting.
Ken Waring

Charles,
Good advice on how to do it but I would find someone with the correct equipment in case things go wrong.
<Axle Stands> are NOT <More Expense>. They are a Cheap Insurance against damage to your car or even worse, yourself. Go without an evenings beer!! Money well spent. I use axle stands but then put any wheel that is removed under the car.
Geoff F.
G. Farthing

Sorry, my mistake: I meant to say " wheel ramps" not axle stands....

I have 2 sets of axle stands & a trolley jack, but I remember that before pumping the car up to 355mm, I couldn't get the jack under the car without reversing it onto some planks of wood.....


So............
Drive car onto small wooden ramps (?60mm high)

release hydragas fluid L&R, replace caps on valves

Jack up the car: one side at a time, and remove/replace pins?


I still have a few weeks waiting, depending on the speed of UK to NZ post.

thanks for the advice, also does anyone know the volume of hydragas fluid per unit?
Charles Dundas

Never measured the volume.
Might be less than a pint for each side.

>release hydragas fluid L&R, replace caps on valves.

I hope you understood why this must be done.
The schrader valves allow air getting through when you do underpressure on the other side. This underpressure will happen when you lift the car from the wheels. So close them with caps.
Dieter

Yes indeed, as they only seal closed when under pressure: no pressure in system & they can suck air in.

I am looking into adding in a small suction pump side circuit to my hydragas pump, so I can drain the fluid, suck out more, then re-pressurise without having to remove the valve connecter.
More brass fittings and valves required for this though......and priming of the pump to remove all air as well..........

Then do the other side, as above.
Charles Dundas

And if the released fluid is like "rosé wine" renew to homemade fluid that is rust inhibiting !!
Destilled (de-ionized) water,regulare coolant glycol and if found (!)pure alcohol is a good mixture.
1/3 of each was fine for me as the original expensive fluid was rotting the metal can it came in ....
Carl Blom

My recipe (basically 50:50 green glycol coolant & methylated spirits)is from here:

http://austin1800.homestead.com/Page11.html

Official Recipe for Hydrolastic Fluid:


The liquid is a solution of 49% alcohol, 49% distilled water, 1 % triethanolamine phosphate and 1% sodium mercaptobenzthiazole, which is of constant viscosity and has a freezing temperature of 31 degrees C. According to BMC, the particular reason this fluid is used instead of distilled water with an antifreeze is that this fluid IS an antifreeze solution of constant viscosity containing a rust inhibitor with an agent added to make the fluid distasteful. (This last was a legal requirement.)
A 50-50 mixture of methylated spirit and antifreeze (with a little radiator anti-corrosion added) can be used successfully with no adverse effects.


There were variations in the formula for Hydrolastic fluid between the UK and Australia. The firm of H.C. Sleigh (Golden Fleece) marketed a fluid to the following specification: Alcohol 50%, Water 24.45%, Bentmazol 0.05%, Ethylene Glycol 3%, (Bomar)? 0.50%, Union Carbide HB5100 22% (this is a viscosity improver).
Charles Dundas

IMO the original "modern" fluid sold in steelcans is not good enough as rust preventive in the system.
Dieter was first to point in that direction several years ago when he cut failed spheres and opend up so the valves etc. could be examined. Also see my picture of the original can that was totally rusted thru..
Polypropylene glycol (the blue stuff that turns green upon dilluting with water) has in itself strong rust inhibitors as well as inhibitors for alu,brass etc.
But it has to be with at least 50% water to get right viscosy and work as intended. Probably it would work without adding any alcohol as the above mixture is also heavy anti-freeze.
Just wonder how a totally empty system would work with Citroen-based fluid ? This fluid is very good, doesent foam ,low freezing point and not harmful to any metal or rubber/plastic material. But the problem is to get any trace of the old water-based liquid out of the system !
Carl Blom

This thread was discussed between 03/08/2006 and 07/08/2006

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