if your talking about the "naked" valk, the trick is to make sure u fully collapse the knee joint, and the knee cap is used to keep the whole leg straight. at the same time the shoulder block's edges' friction on the wing hinge holds the whole valk sturdy.
right now i have 6 completed 1/72 vf-25 models: 2 naked, 2 with super parts and 2 with armor
for super parts version, i agree it was a total farce and i have since junked all the super parts i ever bought. there is still a way to still make the super parts work in every mode, but needs a more detailed discussion than this.
and armored parts... solved every problem there ever was