framer has it. However, to say the PTU 'gets pressure' from system A fluid is not quite right.
The PTU is a pump, which pressurizes part of the B system, this pump is driven by a hydraulic motor, which is operated by system A. There is no fluid transfer, the link is purely mechanical.
This part of the B system takes fluid from below the standpipe, in case there is a loss of fluid from the main part of the system. However, as framer correctly states, if there was no fluid (at all) in system B, the PTU would not work.