It does sound like it could be a coincidence with changing the master. That master is also listed for some disc/drum applications BTW.
The suggestion of fluid contaminated linings sounds plausible, or even something else coincidental like a collapsing brake hose. A step by step diagnosis is in order. It would be rare that the valve body is the culprit, esp. for a pull.
Had some slight contaminated drum shoes cause an intermittant pull like that once. Left when cold, right when hot... so sometimes it would go a little left, then hard right. Most times it was fine. One side showed almost imperceptable signs of grease contamination from a bad seal... kind of a hairy build-up at the shoe ends. Maybe front discs could do something similar. Check it all.