Sometimes it is not so easy to fill the internal reservoir because the tubing to allow the air to come out of the reservoir is very small. This air comes out the OVERFLOW port. If there is any oil in the line from the top of the reservoir to the OVERFLOW port it causes back-pressure in the reservoir and does not allow the reservoir to fill with fluid. You can put a very slight suction on the OVERFLOW port to try and clear any oil and watch if the fluid level drops in the fill tube. Otherwise, open the drawer of the PPCH and disconnect the tube from the top of the reservoir to allow air to escape, and fill the reservoir with this tube disconnected. You can pressurize this tube just a bit to push any accumulated oil from it out the OVERFLOW port. See the below Figure 9 from the manual.
All of the above assumes that you do not have an external reservoir connected to the PPCH. If you have the external reservoir the procedure is different. Contact Fluke Calibration Technical Support at the below link or by calling 1.877.355.3225