Don't even question it if that transmission fluid has been in there for more than two or three minutes. The fluid is contaminated and EVERY part that has rubber parts in it must be replaced, (not the power booster as that doesn't contact the brake fluid).
The steel lines must be flushed with brake parts cleaner and dried. Parts to replace include the master cylinder, calipers, wheel cylinders, combination valve, height-sensing proportioning valve, (most trucks and minivans have them), all rubber flex hoses, and the RWAL anti-lock brake valve assembly or the 4-wheel ABS hydraulic controller. If any part is ignored and left on the truck, that contamination will leach out and recontaminate the entire system, and you'll never have good brakes again.
Every year I did a demonstration for my students to show what happens to the rubber parts. I put a wheel cylinder seal in a beaker of clean brake fluid and another one in clean fluid, then added one drop of power steering fluid. (Any petroleum-based product will work). After one week the contaminated seal had grown by 25 percent and was soft and mushy, just like those in your master cylinder right now. Years ago we even ran into problems when people would repack their front wheel bearings, wipe their hands on a rag, then fill the master cylinder. They would use their fingertip to reset the rubber bladder seal in the master cylinder cap, and the residue on their finger was enough to cause problems a few days later.
Monday, October 22nd, 2012 AT 12:55 AM