with the steering box. I dont belive oil compresses, Thats why when you have air in your brakes the pedal is spongey because the air is compressing when you bleed them the pedal gets hard, But I might be wrong,

 

Well I guess compression is not what I'm talking about, it's pressure. You need a certain amount of PSI to move the ram. The more the resistance you have the more PSI it will take to move it. This is going to create heat. My concern is with a closed loop like this where does the oil expand to when it heats? Like I said, I might be wrong on this and I'd really like to see it tried to know for certain but I think even if you could get all the air out you'd have problems with the pressure build up.

 

To bleed the system you just crack the line when the cyclinder is full (fully extended), then just turn the cyclinder the opposite way to push the fluid out. Or put a T where the hose goes into the cyclinder with an air bleed on the open end. This is similar to bleeding the brakes. I'm always having to do this to bleed out leaky cyclinders on old farm equipment.

For the heat issue, a finned steel line cooler could be used on the lines leading back and forth between cyclinder, but one needs to be used on each line as there will be 2 seperate fuild masses.

 

Your bleeder would need to be on the highest point in the system so you could bleed out the air that will move to the surface naturally. And considering the amount of pressure that a cylinder can handle, would heat expansion really be an issue??? It would theoretically be equal on both sides as long as they are bled of all air.

 

Actually considering the design of the system, as long as the bleeder is at the end on one of the cyclinders, then just bleed the cyclinder on the axle before its installed, as both fluid masses flow through each cyclinder.


This has got me thinking of a different way to run a rear steer. You would need to run full hydro up front with an aftermarket hi-output hydro pump though. The system would involve the same displacement method too, just hook one ram to the steering up front, location doesn't really matter as long as its strong and up and out of the way. Then run some hoses up into the cab for the following valves: a reverses valve to reverse the flow so one could crab walk and such, and a valve to cycle the flow to turn of the rear steer.

Or one could run the hoses from the steering valve to some T's to direct flow front and rear, then up to the same valves in the cab for both front and rear to save money cause electric pumps can be pricey.

One thing i just remembered, all hydraulic hoses are steel braided, they just use a rubber outer coating of sorts to seal and dprotect the braids from fraying.

Also,

 

I knew hydraulic hoses are braided steel, but the aircraft hoses are much higher quaility and have two layers of braided steel. And I am doing rear steer on my jeep like you said one pump, one charlynn valve and I also have a valve that directs the fluid to rear it has three positions normal nuetral and reverse, the only thing is instead of 4 turns lock to lock with the rear steer engage it will be 8 turns lock to lock

 

I forgot about them aircraft hoses. But thats a good idea with the 8 turns lock to lock, on thing i might suggest is a valve to turn the front steering of as well, in case you blow a seal on the ram or a hose, that way the system won't drain itself and leave you with no steering at all.

 

I've been thinking about your bleeding issue, and came up with an idea, mount a pair of tee fittings to the cross member behind & above the upper cylinder, on one side of each tee will have a hose that goes to the lower cyliner, the other side will go to the upper cylinder, and in the third port install a high pressure quik coupler, fill your lower cylinder and hoses then hook them to the tee then your upper cylinder and hoses and hook them up,
keep in mind the fittings need to be on the top side of the cylinders,
now I know you guys already mentioned tees & bleeders but to bleed the system you have to get oil in there to replace the air, thats where the high press. couplers come in, once everything is installed and filled as best possible hook up a porta power hand pump and pump the oil into you sytem, on the tee that is not hooked to the pump rig up another coupler with a valve on it so you can pump up one side to say 25-50 psi and bleed the air out, then switch sides with the pump and bleed coupler and bleed the other side of the cylinder,
once all that is complete and all air is out of the system you can get both your cylinder centered by pumping up on side and bleeding the other, for example your upper cylinder (master cylinder) is 1.5 inches in from being centered while your bottom cylinder (slave cylinder) is perfectly centered, hook the pump to the side of the cylinder that the piston is furthest into and the bleeder coupler on the other side, open the valve and pump up the other side till the master cyliner is centered, when your done just remove the pump and bleeder coupler, the high press couplers work like the couplers on you air hose for air tools so you won't get any air in the system when you disconnect them and they can stay on the tees for maintinance later, keep in mind though that you do not want to leave to much residual press in the system because it will be hard to connect and disconnect the pump & bleeder couplers,
A little complicated but I think it would work good,

 

After thinking a little you may have to block you tirs to keep them from turning while applying pressure, either that or do not use as much pressure,