Hot-start issues are a common problem affecting Power Stroke 6.0L diesel owners.
This problem is almost always caused by the engine's high-pressure oil system having a leak at some location in the system. This system also runs the diesel fuel injectors, hence the issue.

There are several locations where the leakage may occur. They are:

High-pressure oil pump
Diesel injector inlet seals
STC fittings
IPR Valve
Branch tubes
Standpipes
Dummy plugs
Oil rail ball tube O-rings

The system needs to build 520 PSI in order to start. If the leak (or leaks) is severe enough, 520 PSI can not be reached and your truck will not start at all.
This IPR fitting is installed in place of the IPR valve and allows the high pressure oil system to be completely sealed in order to detect and identify any leaks present in the system.