You mentioned new seals fitted as well as the Fabled Orange Clip. Did this include the thin Oring around the filter cap? The old ones certainly go stiff over the years and even a new one can fail if it becomes displaced or kinked slightly during reassembly. Here's mine (not in the fitted position for clarity):
Only a new Oring will cure this situation.
Less than 50psi fuel pressure at the Schrader valve will prevent ignition.
TC