Absolutely correct. The push in fittings are NOT suitable on suction side.
A more severe issue is that the retaining ring, which is heavily spring loaded like a barb, eventually collapses the tubing as well, restricting flow even on pressure side. Regular trimming does overcome this. Must be due to fuel depolymerization of the tube materials as you do not see this using air.
The best of the festo's is the ones with the locking cap, almost like a gland fitting. AMT used to use these on their engines in the early days.( NPCK type)
Andre
