I'm no expert but, I just dealt with the valves for the first time on a recent build for my future father in law. He did most of the leg work but, questioned if he needed these things too. Everyone he asked said that they're a must. Without the gravity assist of a firewall mounted master cylinder, all said that the fluid would drain back into the master easily and potentially create dangerous situations with brake performance.
Again, I'm not expert and neither is he but, we did not want to take a chance. Do you?
It may be helpful in your situation to know, we did have issues with the valves after instal. We had issues with them sealing up. He bought a new one to replace one of the valves. Upon inspection and comparison to the new valve, that was the exact same brand, you could tell the quality of the original sucked. After the new one was installed, no issues but, the truck only has 84 miles on it as of Saturday so; we'll see how they do going forward.
I don't recall the brand we used but, we got them from Jegs. You may want to remove and inspect the quality of the valve(s) you have.