On second thought..... I have a 12 volt electric pump that does power up, gravity down, but it would be really really slow since the cylinders have such a large bore.
The high pressure hydraulic hoses are really expensive too
I think i'm going to use air over hydraulic with about 100 to 150 psi. That doesn't sound like a lot of pressure, but a 4" bore will have 12.56 pounds of force for every pound of pressure on the input side. In other words 100 psi going in will yield about 1,250 pounds of lifting force on the working end for each cylinder.
A 4" bore with a 48" stroke would require two gallons of fluid for full extention. I'll use small diameter tubing and fittings so that in the event of a broken hose, it would take several seconds for all the fluid to be forced from the cylinders adding a measure of safety. I'd like to make the jacuzzi rise to full extention in 7 to 10 seconds.
A safety lockout mechanism is also important. I'll go take a look at the ones used on automitive hoists.
I also need a good source of air.....since this bus has hydraulic brakes.