Yes a spring will definitely be needed… the weight of the car will be enough to collapse the cylinder while its in the air but you’re going to need something to get it the rest of the way once the weight is back on the suspension.
I’m thinking of fabricating a box (1/8" steel comes to mind) right in front of the rear wheel which will protrude up into the car approx 4". welded on top of the box will be cylindrical donut (same exact one used on hydro cars) that will locate the cylinder. We need to design this around the cylinder as we are going to need to weld the cylinder collar to this donut in order to keep the cylinder up inside the car. This will make the actual case and guts replaceable from inside the car should they need to be rebuilt.
inside the “box” on each side of the cylinder will be a bolt in (from inside the car) spring mount where we can just clip on a spring. The spring is going to need to have tension on it when the cylinder is fully retracted. This is going to be the worst part I think, is finding a spring that can sustain that kind of extension.
Design #2 would be a 2 way cylinder out of say a tractor or something with the ability to move both ways under power. It will require a slight bit more fabrication and plumbing but we can still use a single pump 
On 12v no matter what you do this is not going to operate fast at all, in fact it’s really going to come down to cylinder size and pumphead gear / pressure to see if it even really lifts on 12v. There may be a better gear combo than a lowrider pump for this design, we are going to have to look at pumpheads and do some calculations.