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I am getting TnT cylinders sometime this spring. I think what I want to do is to connect the associated valves into the PB loop, but not add the return to tank line, that way i am staying more or less oem. I am looking at 2x Bucher valves with float (7153412), are these the right ones for the job?

Is there value in QDs from the TnT cylinders to the valve stack? I was contemplating pulling the valves off anytime i remove the 3ph for the backhoe, keeping them with the 3ph gear.

Is there a substantial disadvantage for not plumbing the return to tank line? Only rear implements i am currently using are BB, tiller, & brush hog(in the future) (and BH... but its not part of the 3ph)
 

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Your valves need a way to dump the pressure out of the cylinders. When you engage the existing rear ports, one is connected to the pump's high pressure side and the other is diverted to the tank return or sent to tank via the power beyond circuit, but if you engage a valve down-stream and don't have a valve capable of handing the return line pressure (some are as low as 500 psi, some are 1000 psi, while your high pressure side is likely 2900 psi) you'll likely blow out some seals or worse.

I have a 2-spool valve bolted to my fender for my stump grinder's operation, and I've been using the rear hydraulic valve to operate that by manually holding the lever while using the grinder. This provides high pressure from one port and a return to tank through the other, while my 2-spool valve diverts flow accordingly to the two cylinders. I have the valve hooked up via quick connects as a matter of expediency as I didn't want to take the time or expense of properly plumbing it when I put it into service. I have another 2-spool valve coming tomorrow which has float in one of the spools to be dedicated to my top-n-tilt, and that too will be mounted to my fender, but will only have a quick connect for the hydraulic top link. I have no expectation to ever desire removing my tilt cylinder aside from needing repair, but I can see where I might want to not have the top link hanging out back (it's not installed now - I have my logging winch on the machine). When I do my permanent plumbing of both of those valves, I'll add them into the tractor's hydraulics and they'll be plumbed into the PB and tank return ports accordingly.
 

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Here is the Bucher part numbers for the Rear SCV that I have.

http://www.greentractortalk.com/forums/medium-frame-compact-utility-tractors-mcut/144457-question-about-3rd-4th-5th-rear-remote-install-4110-a.html#post2481705

Here is how i installed the rear SCV on my 1025R.
http://www.greentractortalk.com/forums/fit-rite-hydraulics/114898-fit-rite-hydraulics-rear-selective-control-valve-installation-1025r-oem-power-beyond.html

As Jim Timber said, valves are not designed to see pressure on the tank port. If you connect you rear SCV to the PB on the 1025R, this set up will work, although, the PB on the 1025R has a pressure side and the other port is the feed for the rockshaft lift cylinder which can be used as a tank port if the 3 point is not lifted.

This design of the PB works on the 1025R because the intended purpose for the PB is to provide hydraulics to the BH. When the BH is attached, the 3 point is removed so there is no way to create pressure in the rockshaft lift circuit because the 3 point is removed.

If you have the 3 point installed, and you have implements hanging on the 3 point, when you lift the implement, the rockshaft lift circuit will see pressure. Again this is the same circuit that is used for the tank line off the BH, but remember, when the BH is attached, this line cannot see pressure.

So, what will happen when you connect the tank port of your rear SCV to the PB. Again, the one line will feed the pressure port on your rear SCV. The tank port of your rear SCV will connect to the PB return which also provides flow and pressure to the rockshaft lift. When using your top n tilt, you will obviously be using 3 point implements. When you lift the implement, the pressure that it takes to lift the rockshaft will be seen at the tank port of the rear SCV which will cause the rear SCV valve to leak.

Your rear SCV should be set up with three lines.
1. Pressure port which can be feed by the PB pressure line.
2. PB port off the rear SCV connected to the return tractor PB line.
3. Tank port which must be plumbed into an added return to tank line.
 
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