Hydrolastic and Hydragas suspension

Copyright © C W Lees. All Rights Reserved.

The following email was received in April 2004 and is typical of those received from Maxi Owners experiencing difficulties with their cars hydrolastic or hydragas suspension systems. The ensuing article hopefully answers most of the questions asked.

Hi: I’ve recently acquired this Maxi.

I drove it the 120 miles home from Poole in Dorset to my house near Weston-super-Mare, and as I entered my village the rear end started to sag. The previous owner warned me that the suspension units were not as robust as they might be. The car has 11 month's MOT and is taxed and insured. Now the rear-end is right down - almost touching the wheels and driving it is "interesting". I do not consider it safe to drive and it is now off the road.

What needs attention? Is it the hydragas units? The car came with 2 complete sets (both looking quite elderly) and the Haynes manual suggests that they can be pumped up. I am not a mechanic so I would be grateful if you could advise me as to what the problem may be, whether it is fixable by the averagely competent garage and what the likely cost is. Graham Bachtold.

History and Technology

For more information about both technologies go to:

If you are interested in learning more about the man who designed the systems then go to
www.moultoneers.net *

* The last named website is run by the Moulton bike club. When visiting the site look for "Dr Alex Moulton" on the site's Menu and then click on it for information about the Moulton family.

The hydrolastic and hydragas suspension systems were developed and then used by BMC/ BL/ Rover in the 60's, 70's, and 80's.

The system developed by Alex Moulton has been fitted with varying degrees of refinement to BMC/BL/Rover Mini's, 1100's, 1800's, Metro's, early MGF's and of course the Maxi (to name but a few). In all 11 million vehicles have been fitted with variants of these systems.

Early Maxi's were fitted with the hydrolastic suspension system, later models had hydragas units fitted.

Vacumn Hydro Suspension Pump supplied by Liquid Levers

Drawing of the Hydrolastic service unit used by BL appointed garages (BL part number 18G 703)


According to Haynes Workshop Manual owners should check the trim height of their car every 6 months (or 6,000 miles). This is a fairly simple matter.

“Place the car in an unladen state on level ground. Check and if necessary adjust the tyre pressures. Bounce the car several times at the front and rear to allow the suspension to be in its normal ride condition. Measure the distance between the top of the wheel arches and the centre of the hub-cap. If its more or less than 14.5 ± 0.25 inches then the system needs re-pressurizing/ de-pressurizing.”...........Source: Haynes Workshop Manual.

If your car suspension does need a pump-up you are advised to go to a garage that specializes in servicing Rover vehicles and or who have the necessary equipment. You should have no difficulty in finding such a garage as Metro's and more recently MGF's were fitted with such systems.

If you do decide to do the work yourself you will need the equipment and fluid to do so. For details on how to go about re-pressurizing/ de-pressurizing the cars suspension system you are referred to the appropriate equipment/ workshop manuals.

To date I have identified the following sources for suspension pump up units.

Liquid Levers Innovations Ltd Fife, Scotland Tel: 0044 1 333428401 www.liquid-levers.com. This company says that its products are Rover approved. The unit shown on this page was retailing at £390 in October 2003.

Ron Hart Tel:01598 763572 email:millfarmhouse@hotmail.com
Cost is advertised @ £75 plus £5 P&P. Click here for members comments on this machine

e-bay / private adverts in car magazines like Practical Classics. Second hand units such as that shown opposite (BL part number 18G 703) are occasionally advertised.

New fluid is readily available from MG Rover dealers, Brown Brothers and Comma oils stockist's. For your local branch see your local Thomson directory or Yellow Pages.

If you do decide to do the work yourself do take care as the suspension system is highly pressurized.

Members tips

The fluid used in the suspension systems of our cars is something we probably give little thought to, but perhaps we should. If you compare fluid removed from a car with new fluid, you may find that the drained fluid has a muddy, red appearance while the new fluid is green and nearly clear. My suspicion is that over the years the original fluid either looses its anti-corrosion properties due to old age or through the car having been 'pumped up' with water, or watered down fluid, leading to extra corrosion within the system. In bad cases I understand that the damping properties of the displacer can change due to the damper valve being held open by the rusty sludge - this can usually, but not always, be cured by removing the displacer and rinsing it out thoroughly. I suspect that it may be a worthwhile precautionary measure to the long term life of the suspension pipes and displacer's to drain the fluid from time to time, perhaps every 10 years, replacing with new.

Phil Marrison

Common Faults

Our Technical Officer Bryan Winn writes :
"Hydrolastic and Hydragas units are linked together using piping which runs front to rear on each side of the car. The pressure within the the hydragas system is approximately 350/lb./in², giving a front trim height from the centre of the hub to under the wheel arch of 14.5± 0.25in. When the trim height is not correct a search for the reason must be found.

First have the system pressurized at a garage which has the facilities. From this point the fault can be found. Observe very carefully any fluid leaking from the system. The interconnecting pipes could be punctured - look closely where the pipes are clipped to the under body. Check the flexible pipe from the interconnecting pipe to the hydrolastic/ hydragas unit. Look for a leak from the unit itself.

Pipe punctured?

Unit leaking?

Flexible pipe leaking?

"Should the hydrolastic suspension system suffer damage and the fluid be lost, the suspension arms on the damaged side of the vehicle will contact the bump rubbers at both front and rear. In this condition the car may be driven with complete safety up to 30 mph (50 km ph) over metalled roads to the nearest distributor or dealer". (Source Austin Maxi handbook).


Hydrolastic unit as fitted to early Maxi's

Drawing of the later hydragas units that were fitted

Both Hydrolastic and Hydragas units are in short supply and can be quite expensive. The only current known source apart from the club and its members is Earlpart based in Derbyshire - www.earlpart.com At the time of writing Earlpart charge between £35 and £130 per unit depending on whether they're second hand or new. Other sources include scrap yards, private advertisers in Car Magazines like Practical Classics and e-bay. Please note that you may have to wait a long time for one to come up for auction on ebay - but come up they do.

Pipework can sometimes be made. For details contact the webmaster.
New fluid is readily available from MG Rover dealers, Brown Brothers and Comma oils stockists.
The relevant part numbers for new units are listed below.


Part Numbers


Chassis Number




145S 101M to 41479M




145S 41480M to 49230M




145S 49231M to 51000M




145SC 101M on




145S 101M to 49230




145S 49231M to 51000




145SC 101M on

Members tips

The most common failure of Hydrolastic displacer's is that the flexible connecting hoses fail. The original hoses were crimped on, but it is not possible to fit a new hose like this. The pipe you need to crimp to is shrouded by the body of the displacer. This makes it impossible to get the crimp tool into the correct position to deploy it. A solution to this problem has been found by a Tony Wood of the 1800 Club' by using a clamp fitting rather than the crimp type. Contact details for Tony are available from the webmaster.

Phil Marrison

The future

Reports are being received that Dr Alex Moulton has found a way of reconditioning Hydragas/ Hydrolastic suspension units. I understand that Dr Moulton has promised to hand over his research to classic car clubs whose vehicles are affected so that they can make appropriate arrangements.

The small print: Whilst we hope that this article has been helpful, we cannot guarantee as laymen that any of the information/ views/ opinions expressed on this or indeed any other page on this website are accurate and or or safe. For this reason we recommend that you consult a competent qualified person(s) prior to undertaking any task on your vehicle.