So I was reading the documentation on the Summit site. Then looking at the photo you posted of your valve setup. I'm going to copy and paste what they have on their website. I count 7 lines on your valve. If that is correct you have it plumbed wrong according to what they have on their website. Here is their verbiage about it. It appears you are connected at the tank fitting and if you actually have a "PB" port then you are not getting the pressure and flow needed for the backhoe and 3rd valve. I may be seeing it completely wrong and if so tell me. It's early yet and I have only made a few mistakes so far today so I haven't hit my quota yet.
How do I find my power beyond on my loader valve?
"Typically, there are seven hoses or steel tubes connected to the loader valve. If only six hoses or tubes; there is no Power Beyond Port on your loader valve. The 1st four are connected to your loader for the up, down, dump, and curl functions. The fifth line comes from your pump to feed your loader valve. The sixth line goes directly to your reservoir so that when the pressure relief valve opens the fluid is dumped directly to the reservoir. The seventh line is connected to the “Power Beyond Port” on your loader valve, which is used to power other valves beyond the loader valve. On some loader valves there is a “PB” stamped on the valve next to the power beyond port. Some loaders indicate their power beyond port with green tape or a green mark on the hose or valve. Some loader valves have a power beyond sleeve inserted in the power beyond port. This sleeve looks like a nut; however, it is 3 to 4 times longer than a normal sized nut. On some loader valves there are no clues to help determine which connection is the power beyond port. Your owner’s manual may not even indicate the location."