Adobe has been notorious for lack of "backward-compatibility" when it comes to brushes.

you need to create brushes in the earliest version of PS that you have access to (since you can only really have "one" version of PS on your system according to the EULA...)

or

stamp out your brushes in BLACK on to a layer allowing white space around it (on separate layers if you have multiple ones...) and save the file. have the person with ver7 open the file and "re"define the brushes in that version of PS and save the set.

this is the "same-lame" workaround for any version after ver6 when dealing with brushes made from a higher version - backward.

retrocity