So since the Diminished Chord Button on your accordion produces 3 notes when pushed you can use this to create other chords.
http://www.accordionpage.com/chcomb.html
Also some accordions do not have a Dim button. To achieve a Dim chord on an 80 bass box try to use C note button and F7 chord button to get one. This sometimes works if the 7th chord is also a 3 note chord. Sometime the 7th button produces a 4 note chord and it would not sound correct. You will know right away. Also one accordion I have has a 3 note 7th chord and its rootless. so C7 has no C in it! Then you can use the E counter bass button and the C7 chord button to create an E Dim(ish).