Brakes Hissing

[Gaz B]

The last couple of days I've noticed a hissing sound when I apply the
brakes, a bit like air rushing out of somewhere. What causes this? I
couldn't find any references to hissing in the archives.
Car is a 1988 Audi 100 2.0, I get the low brake pressure warning lamp
ocassionally but had put this down to a bad bomb (which I know needs
replacing). The brakes do work satisfactorily and the fluid levels are
all ok. There's no evidence of any brake fluid leaks from a quick visual inspection.

Thanks, Gary