Have the same problem with my 92 F450. It appears that the return spring on the release handle rod has failed to force the ratchet to re-engage properly. My temp fix is to always remember to push in on the release handle while setting the brake, then test that it stays in place. The other "fix" was to adjust the brake so engagement happens in the upper 1/3 travel. The real fix will be to take the whole [interior foot brake assembly] apart and either replace it or fix what's broken. Good luck.