Do a search on "brake fluid flush" or something similarly worded. You'll get numerous responses with 'how-to's', to include the old two man method and the modern power flush (home version
As for your initial query, I don't know, sounds a bit like brake fade, except for the grabbing at the end. I am well versed in the effects of brake fade, have never experience the grab you describe.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.