PBL, the PTU transfers hydraulic pressure (via shaft power) between the G and Y systems. There's no fluid transfer.
IIRC, PTU logic is based on p/b, the delta p between G and Y being >500psi and a few other things mainly related to engine start, cargo door operation, flt etc. Reservoir low quantity plays no part, so if either the G or Y system has a leak the PTU will start once the pressure in the leaking system is below 2500PSI and won't stop until the PTU p/b is pressed.