If you do not concern yourself as to the system set up in regards to temperature and pressure, you do not need to worry about the internal mechanisms of the keg.
If you eventually set your system up correctly and you still have issues, then you may suspect the keg's valve at the neck or the internal tube. Although keg issues are rare, system set up or also referred to as "system balance" issues are a lot more common.
Beer has gas in solution! You have to gain perfect equilibrium such that this gas does not want to leave solution nor absorb any excess gas from your pressure source. Thus, temperature / pressure.
Scott Zuhse, Instructor
Micro Matic Dispense Institute