Kenny is right as usual.
We are fortunate to have him and his hydraulic expertise.
Although plumbing hydraulics somewhat follows your thinking, unlike say a sprinkler system that still needs pressure to run it, hydraulic cylinders need lots of pressure to move them. If you tee off both the pressure and return lines to feed your rear remote valve (which you need to get) it will create a "by pass" route for the fluid to go. So when you try to use your loader valve, (which requires pressure to move the cylinders, the fluid would find the path of least resistance and just run through the "T" and through the rear remote valve ...then return to tank. When you go to use the rear remote valve, the fluid would end up running through your loader valve back to tank. In fact, none of your valves (3pt included) would work now because you have created a "by pass" situation in the hydraulic system like Kenny pointed out.
The valves we use on our CUT's are generally "Open Center" valves so fluid is always flowing through the valve until you activate the lever to send the fluid through a work port to the cylinder. Now, the pressurized fluid has no where to go except to that cylinder, thereby moving it. If the pressure gets to great for the cylinder to move anymore, it starts to go through a "pressure relief valve" usually found on one (or all) or the control valves such as your loader control valve. The fluid finds it's way to the least resistance again, activating the pressure relief valve and goes that way back to tank. The pressure relief valve is a safety for the hydraulic system so your pump (or line or seal) won't blow up under too much pressure. Your pump is constantly trying to pump fluid and it needs to go somewhere. The relief is usually set to a pressure that still allows your hydraulic cylinders to operate to their safe operation pressure before it starts to relieve.
In order to put another control valve into the system like you want to do for your rear remotes, you need to use the "power beyond" from the loader valve or another one and plumb that into your new valve. Your new valve will need a return to tank line (or sump) and that part can be teed back into another return line. The loader valve where you took the power beyond pressure from will need a return to tank also. If your new valve has to feed another valve down the line, you need to use the power beyond port from it, just like you did from the loader valve. Your new valve will still need a return to tank. All this type of plumbing allows the pressurized fluid to be directed to the work ports (and to the cylinder) that is being operated by that control valve.
So in a nut shell,( a big one
) that's how that works.
I'm sure there are some things that I've left out or can be explained better than I did, and maybe someone else will chime in with that information for you.
