Citroen dispatch brake problems?

I have an intermittent problem when braking gently - after a few seconds the pedal goes hard and there is a complete loss of braking. The brakes seem to always operate effectively otherwise - the problem only seems to occur now and again and always when there is an extended gentle braking action - eg when approaching stationary traffic/junction at slow speed. Despite having had the braking system overhauled by a reputable garage the problem still occurs now and again. Any ideas????