740 Brakes beginning to squeal

Hi all,

What causes the brakes on my 1990 740 wagon to suddenly begin to squeal,
quite loudly in fact ??

It has been happening for some time in backing up but not nearly so loudly.

Wha hoppen ??

Thanks much,

Perk >)

 

pad type soft braking lots of dust not washed off

 

Thanks John.

Perk >)