My Guess
I'm no expert, but here's my understanding concerning the 757 PTU.
The short story (as I understand it):
High-interest items are on the 757 left hydraulic system. The 757 PTU backs-up (replaces) the left engine-driven pump (EDP) when its output is insufficient, thereby ensuring the operation of the high-interest items.
The 767 has the high-interest items on the Center Hydraulic system. The two AC motor pumps (ACMP) are backed-up by the Air Driven Pump (ADP). Therefore it does not need a PTU for a back-up.
The longer story:
The 757 left hydraulics, which has flaps/slats/gear/steer, utilizes an EDP, (primary), and a supplementary ACMP. If you shutdown the left engine, and therefore the left EDP, the PTU will come on-line to assist the ACMP to operate these high-interest items. The 767 center system does not use an EDP, but rather two ACMPs. As the ADP backs-up the ACMPs, a PTU is not necessary for the 767.
Having said that, the 767 does in fact have a PTU. It is used as a back-up for stab trim.
POWER TRANSFER UNIT (757)
Control
Loss of left Engine RPM (below 51% N2) or a drop in Engine Driven Pump
(EDP) output pressure below 1200psi, with the right Engine running, will
complete a circuit to open the PTU Control Valve. If pressure is not available to the PTU CONTROL PRESSURE switch within three seconds, the PTU on-relay will energize closing the control valve. With the left engine running and with high EDP pressure, output power is available to the close side of the control valve. Operation of the PTU switch on the overhead panel (P5) will provide power to open the Control Valve.
Indication
The PTU will display as left system pressure on EICAS with both left pumps off.