Cylinder mouth O-ring receiving groove Sharp edges corners and threads Soft thin-wall metallic sleeve Installation tool Sharp edges and corners O-ring receiving grooves Threads area O-ring receiving grooves Paper cover Paper entering sleeve Sharp edges and threads O-ring receiving grooves Installation tool push type Internal O-Ring Installation (using metallic sleeve to avoid O-ring damage from sharp edges or threads and push-type installation tool) Internal O-Ring Installation (using paper entering sleeve to avoid O-ring damage from sharp edges or threads and push-type installation tool) External O-Ring Installation  (using paper cover to avoid O-ring damage from sharp edges or threads) A C B Figure 12-61.  More O-ring installation techniques. Filter Units Pumps Power Transfer Unit (PTU) 12-40 Refer to  Figure 12-64  for the following description. Both A  and B hydraulic systems have an engine-driven pump (EDP)  and an ACMP. The system A engine-driven pump is installed  on the number 1 engine and the system B engine-driven  pump is installed on the number 2 engine. The AC pumps  are controlled by a switch on the flight deck. The hydraulic  case drain fluid that lubricates and cools the pumps return  to the reservoir through a heat exchanger.  [Figure 12-65] The heat exchanger for the A system is installed in the main  fuel tank No. 1, and the heat exchanger for the B system  is installed in the main fuel tank No. 2. Minimum fuel for  ground operation of electric motor-driven pumps is 1,675  pounds in the related main tank. Pressure switches, located  in the EDP and ACMP pump output lines, send signals to  illuminate the related LOW PRESSURE light if pump output  pressure is low. The related system pressure transmitter sends  the combined pressure of the EDP and ACMP to the related  hydraulic system pressure indicator. Filter modules are installed in the pressure, case drain,  and return lines to clean the hydraulic fluid. Filters have a  differential pressure indicator that pops out when the filter  is dirty and needs to be replaced. The purpose of the PTU is to supply the additional volume of  hydraulic fluid needed to operate the autoslats and leading- edge flaps and slats at the normal rate when system B EDP  malfunctions. The PTU unit consists of a hydraulic motor  and hydraulic pump that are connected through a shaft. The 