For many mini owners, you either love Hydrolastic suspension or you hate it. Many of my older friends with minis grew up with dry suspension in 850’s or early Coopers and they absolutely loathe hydrolastic. Me, my first mini was a 67 Cooper S and I love hydro.
However, even though I love hydro, I have driven minis with hydro that were too soft in the suspension for my liking. Even though the minis had their suspension evacuated and pumped up by mini specialists with the factory style machine. The reason is because there is air in the hydrolastic system and this air has been trapped in the displacers. I firmly believe that once you get air in the displacers, there is no way even the factory style pump can evacuate all the air from the displacers. You may be able to remove it from the rear displacers, but I believe there is no way it will remove from the front, especially if the displacers have not been filled with fluid when they were installed.
In one of the many posts regarding the hydro system on this forum, I recall Dr Mini stating that when you reassemble the hydro suspension that you must fill the displacers with fluid before assembly. I believe this is essential when you reassemble the suspension. I honestly cannot see how you can expect to evacuate at least 185ml of air from each of the normal mini front displacers, minimum of 250ml in Cooper S displacers. In a past life I worked in a bitumen laboratory and used vacuum pumps on many occasions. I just cannot see that it is possible to evacuate that much air and expect the hydro lines and the displacers to be backfilled with fluid. I could be wrong, but that is what I believe. As a minimum, you need to have the front displacers reasonably full of fluid so it can be pulled back with the vacuum pump.
Even though I did not have access a factory style pump, I found a way to reassemble my suspension with a plumbers water testing pump as used by some people on the forum. I was clinical with trying to eliminate air in the displacers as well as the system, even though I did not have access to a vacuum pump. The result is a car that has nice firm suspension, just as it should be.
I spent considerable time cleaning my displacers over many months and was amazed at the amount of crap that kept coming out. During the cleaning process, it became very obvious of the resistance in the displacer when they are full of fluid and when they have air in them. When the displacers are to be filled, it is essential they are filled in two sessions. When you think you have filled the displacer in the first session, you need to work the displacer to ensure fluid gets into and fills the lower chamber. You will notice the displacer gives a bit of a gulp when you work the displacer and the resistance in the displacer increases dramatically once it is filled.
I filled my displacers to the top of each hose and sealed them off. When I reassembled my suspension I was obsessed with eliminating air as much as I could. The problem on the rear subframe is that where the displacers connects to the steel hydro line, the connection is lower the top of the displacer. When you go to connect it, you loose fluid from the hose. When the subframe was out of the car, I practised and practised joining the displacer hose to the hydro connectors at the rear, until I was confident that I was losing as little fluid as possible.
Before I assembled the rear displacers to the hydro line, I filled the steel hydro line with the plumbers pump and ran a return line to the pump using clear plastic ¼” hose from the front unions back to the tank of the pressure pump. The 1/4" hose fits neatly into an old displacer hose. (See attached photo). I kept pumping until no air was coming back to the pump. Once I was happy that I did not have any air in the line, I connected the displacer hose to the steel hydro line at the rear of the car, as quickly as possible to eliminate loosing fluid. Once the hose was connected, I raised the arm of the suspension with a trolley jack which pushed any air trapped in the rubber hydro hose into the steel hydro line. I then pumped until I could see that the air had come through the steel line and the plastic ¼” return hose and was pushed back to the pump. Under slight pressure, I then lowered the trolley jack slowly until the rear suspension was at full droop. I actually stuffed it up on one side, got air in the LH displacer and ended up dropping the rear subframe, so I was sure that I refilled the displacer. Probably should have just removed the displacer, but I was so peeved off at the time, I just took out the subframe and started again.
When it came time to do the front, I had ideas of using a similar method of lifting the suspension arm and expelling the air through the loosened union at the front. However, I thought that I would not be able to raise the suspension quick enough to ensure the hose was still full of fluid. I imagined that it would probably fall out of the hose, quicker that than I could raise the front suspension. So I abandoned that idea.
I practised and practised joining the rubber hose connector to the front union as quickly as possible. In the end, I got it down to losing about 5 to 7ml of fluid during the connection process. I used a 100ml plastic graduated flask to fill the displacer hose and could see what volume it took to refill the displacer hose. As long as the 5 to 7cc of air stays in the top of each front displacer hose, I reckon that it will be good enough.
I made up my own brew of fluid with a 50/50 mix of glycol concentrate and distilled water. I was thinking of trying a 60/40mix, but found that I could hardly move the suspension arm when the displacer was filled with this mix. I thought that 50 year old displacers may not like taking the extra strain of a thicker mix, so I left it at 50/50.
I am very happy with result and my suspension is nice and firm. I believe it is as good as it could possibly be. To date, I have not fitted shock absorbers to the front and am keen to see what difference it makes. I have installed competition bump stops to the rear.
I think I paid about $60 for the pump on ebay, another $60 for the Schrader valve and $20 for Pirtek to connect the Shrader valve to the pump hose. It reckon it was the best $140 I have spent on the Mini.
Sorry for the long winded story.
Regards Fritzie
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