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... He said he used Valvoline Synthetic brake fluid.
That's a DOT 3/4 fluid, I've used a bunch of it; pretty good stuff. I think I would try to pull a wheel when the problem occurs and see what happens when you crack the bleeder valve. A dribble is OK, a big spurt would support your theory that something is pressurizing the brakes. No one's said anything about the master cylinder, or maybe the electronic brake bias system if your friend's car is new enough?
Thanks Bill & DrCoffee, that sounds like the plan I had in mind!
BTW: I was wrong earlier when I said he used silicone brake fluid. I just went there when he said synthetic. I just checked the Valvoline website and this is what they say: SynPower High Performance Synthetic Brake Fluid is recommended for ABS, hydraulic drum and disc braking systems and hydraulic clutches requiring DOT 3 and DOT 4 fluids.