pinging maxpower

hey Glen, 2003 Grand caravan. friend has one with front brakes bad.....wife
driven so who knows....lol anyway 44,000 mi, fried pads, rotor, broke the
piston in the left front caliper. he was told the piston doesn't come
separate.....so he changed out the entire caliper. pedal goes to floor
obviously until bled but he bled it 12 ways to Sunday, still went to the
floor and has no more time to work on it, so took it to the dealer.

Now they attempted to bleed the entire system, and got the same results.
They attempted to order a master cyl. but got a red flag asking for call to
tech. After they called and stated what was going on, the guy at tech
said.......let me guess, the left front caliper was replaced, wasn't it?
after telling him yes, he proceeded to tell the dealership person that
there's an issue with the 2003's. if you get air in the system, it destroys
the hydraulic assist unit and it must be replaced. So they ordered it, and
told my friend they would have it done tomorrow. He called me tonight and
asked if I had heard of a hydraulic assist module. I hadn't, but not
turning wrench full time anymore, so there's lots of new stuff out there I'm
unfamiliar with. he states 900.00 including labor to replace it and turn
the rotors.

any ideas what this unit is they are replacing for him ?