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

Congratulations on your attention to detail.
Regarding trapped air, it's a great pity that BMC didn't follow the Morris 1100 design with the fill valve at the high point of the front hose.
I'm tempted to make a pair of front hoses with a tee and bleed valve there.
I just blew a rear bag on my Matic, after 51 years!
I had some prefilled and tested spares so no problem.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Morning Chaps,
Have always ran comp' bump stops both front and rear on my hydro', with shocks on front as well.
Ride becomes very "loose" for want of a better term when front stops are resting on subframe.
Hence, I have ride height about 1 cm off front bump stop but in reality, car stance suffers and height is particularly high in the rear.
I was pondering whether to return to the standard bump stops for the front which will allow lowering of the vehicle to a more acceptable height.
Anyone had any experiences in this area?
Thanks
Peter

When my original rear hydro bump-stops fell apart, I tried to fit competition bump-stops all round but ride height was excessive, which is the purpose of competition bump-stops anyway.
I ended up using standard front bump-stops and cut-down competitions on the rear to get standard ride height.

Peter,
Since my last post I have installed shock absorbers on the front and wow, what a difference it makes to the ride. No bounce, just nice and firm. I only have standard bump stops in the front and it seems to work well.

I have always ran my cars fairly low, however I have been persuaded by a friend of mine that if I wanted to get the full benefit of the hydro, that I should try increasing the pressure. The pressure gauge on my red pump reads way out, so I just increase to the height that I want. To give you an idea of the stance, the attached photo has the front at 305mm from centre of wheel to the flair and the rear at 315mm. It is just touching the rear bump stop at this height.

I have since increased the height by 5mm all round. I can just get the end of my little finger on top of the rear bump stop, probably 8 to 10mm clearance. At first I thought it was sitting a bit high, but I am getting used to it.

To partly answer your question, this is with standard bump stops at the front.

Great report Fritzie on Hydro issues. To add to your great information I recently restored the rear end of our 70 S and made up a pump from a plumbers pump as you did. I also added a bypass valve (from Reece plumbing and connections to the schrader valve from Cram Hydraulics) just past the shrader valve so that I could maintain fluid in the pump hose, but allow evacuation of the displacers and pipes (see photo). I also connected a flexible water pipe to the bypass and put the end in a large bottle with the end covered in hydro fluid so I could see any air expelled from the system. I only did the back end, but what I found, as you did, was despite filling the displacers up to the top of the pipes and filling the steel pipes to the front, I couldn't connect the displacer hose to the steel pipes fast enough to avoid air in the system. I also couldn't find any fittings that I could use to temporary seal the hoses and pipes while refitting the displacers. What I found was that if I jacked up the back of the car as high as my floor jack would go, then I pumped up the system, then released the pressure through the bypass valve and left that valve open for at least 15 seconds I was able get rid of the air. The raised rear end of the vehicle meant the schrader valve was then the highest point in the rear end of the system, so air would migrate to that point. I pumped up each side and released the pressure through the bypass valve twice on each side and found on the first pressure release a lot of air was released. The second pressure release resulted in very little air coming out of the system. Dropped it to the floor and then adjusted the ride height each side. The front end is a further complication as has been pointed out. Suspension is now great and have packed the pump away for next hydro repair.

Bazzz

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Mini Cooper S Mk 2, 1970,
Mini van 1970 with side windows now converted to 4 seater traveller