Sagging of the front suspension in hydro cars is common. I believe the bags also contain a rubber “spring” which deteriorates with age. More weight on the front bags (and internal rubber springs) means they deteriorate more with age and thus the front sags more than the rear.
This supports the comment from Mick above, that second hand rear bags may be better than front ones.

All hydro cars sat a bit higher at the back when new. Even a Cooper S. They had to, to cope with passengers in the back + the fuel load.

re the rubber spring comment, yes it does sag a bit with age but it doesn't deform like dry suspension rubber cones do. The rubber spring is at the top of the displacer, when you bounce the car you can watch the hose move slightly.

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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.

I believe my hydrolytic bags are worse for wear and most or some, will need repairing or replacement.
Notice it is not possible to buy them new anymore. Not keen to replace them with rubber cones.
Any suggestions? Mine is an ordinary 66 Deluxe. I live in Sydney.

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1966 Morris Mini Deluxe
.... 4 bicycles and 3 screaming kids.

When you say, " they are worse for wear ", are they leaking , are they sagged in the front , or is the car sitting too low . All of these problems are fairly easily fixable depending on the actual problem .

I haven't gotten a 'professional' diagnosis yet and added to the fact that the car has been in hibernation for 7 years now. While the car was being driven a decade ago now, the front suspension would inexplainably rise producing a loud knocking sound. This would go on for a couple of minutes then settle down again. Tried to find out what it was but no one at the time had a clue. Made the 'mistake' of listening to a NW Sydney mechanic who told me he'd crimped the hydrolastic pipping separating the front from the back. I accepted this, and now not sure if this was the way to go. It fixed the problem.
After that the ride became harsher, but it was too late.
Also noticed the bags needed pumping up more often and it was more difficult to maintain the recommended height (centre of front lights to level ground) as before. I assumed the bags were leaking.
I would like to restore the old car back to its original condition. Part of this would be to restore the hydrolastic system to the way it was. Any suggestions who I should see and/or suggestions how much money this would involve?

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1966 Morris Mini Deluxe
.... 4 bicycles and 3 screaming kids.

The knocking sound may have been caused by one (or both) front displacers becoming dislodged from the locating lugs in the subframe. If this has happened, it could be an expensive fix. With the now butchered hydro lines, no thanks to that dopey NW Sydney mechanic, it seems to me a full overhaul is necessary to get the car right ie lines replaced, displacers checked, subframes lugs (at front) checked, ball joint cups renewed. Unless you can do the work yourself & have access to a hydro pump, the cost could be enormous. To replace the hydro lines, rear subframe has to be dropped & front one lowered. I just hope all is not as bad as above & you can get it fixed economically.

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1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.

Bugger

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1966 Morris Mini Deluxe
.... 4 bicycles and 3 screaming kids.

Maybe , just maybe , there might be someone on here that is closeby to you that could give you a diagnosis on what you need to get your hydro suspension right again .

Personally , i have never heard of the noises that you describe , and yes, i think the mech that crimped the lines off didn't do you any favours , by doing that , he has given you no suspension at all .

If there are some people here who care for, &/or like, & want to retain their
Hydolastic "Ride Quality" ,,, then you may be interested to know that
i prepare a set of hi-los & rubber donut "package" that closely mimics the ride quality of the hydro cars
I call it my "road-soft" set-up
works a treat & very very comfortable ride.
saves having bags blow on you while you`re enjoying driving your lovely mini , retaining reliability at the same time

However,,, it`s not very good for racing on bitumen,,, really really good for wet tarmac & dirt track events tho
anyways,,, just saying, for the sake of those who like the feel of hydro but don`t
want the dramas of the old bags

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.