My car has had a light judder when accelerating on low or medium throttle for some time.

The air bag on the front left strut gave up the ghost so I bought a new bag. When I stripped the strut to replace it I found that the strut had also failed.

I assumed that replacing the whole strut would resolve the problem but it remained. A few months later I wondered if I needed to calibrate the suspension heights. On the first drive after the recalibration the air bag failed on the right leg.

Having now replaced the whole strut on the right hand side the judder appears to have gone.

I can only assume that the strut had been leaking for some time and the air bag was doing all the work.

So my question is hoe do you identify a problematic strut when you cast only see if it is leaking by removing the air bag from it?

Personally I work on the basis that if a strut is over 80K miles old then it'll be well past its best and need replacing regardless of whether it's leaking.

Since the airbags seem to last somewhere between 80-100K miles I would just assume the struts need doing at the same time if I couldn't prove they'd been replaced for some other reason. 2012 (L322) 4.4TDV8 Vogue SE, RSE, Fuji White over Arabica.
Previous V8s: BMW 645Ci, P38 4.6, P38 4.0, Merc CL500
Previous others: Honda x1, Volvo x7, Rover x6, BMW x6, Ford x1, Mercedes x2, Skoda x3, VW x2, Renault x2, Citroen x1, Mazda x3, Nissan x1, Audi x3, Kia x1, Fiat x4, Subaru x3, Austin x2, Triumph x2, Jeep x2, Hyundai x2, Lexus x1, Mitsubishi x1, Saab x2... and some others I've forgotten!

The joys of not knowing how old some parts are.

I think the front left hand strut was damaged by a pot hole and the air bag eventually gave up.

The right hand strut looked like the air bag was just old and perished.

You are right, I think it is a question of when rather than if.