1993 GMC Suburban soft pedal

Had all brakes relaced and several brake lines, and master cyl.(2 Yrs ago) Pedal will over time get"soft" go to floor and cause brake warning light to come on. Each time( every 3- 4 mo) have brakes bled and am told no leaks in system. Brake will be fine and gradually return to "soft". System has a unusual brake device that is connected to the 2 bowl master cylinder, then a seperate line originates there for each wheel cylinder. It is an aluminum casting with fins and has at least 2 bleeder valves. The mechanic will not touch it. I think there is air trapped in it even after bleeding the lines. Please provide me a suggestion or 2. Thanks Tony