DOT 3 and 4 brake fluid, and DOT 5 silicone, are compatible with the SPECIAL rubber used for seals, O-rings, and hoses of brake lines. These fluids are non-petroleum products.
DOT 3 and 4 may or may not be compatible with rubber products intended for petro-based fluids (engine oil, tranny fluid, PS fluid, gasoline, many workshop solvents).
Petro-based fluids are NOT compatible with rubber intended ONLY for DOT 3 and 4 fluids. This includes the O-rings in MC's and Proportioning Valves.
Contact of a petro solvent with these seals will destroy them. In case of accidental contact, clean immediately with detergent-water, then follow with Brake System Cleaner.
Don't ask how I know this.
Since the rubber 'booties' on caliper pistons and wheel cylinders may contact brake fluid (from their insides) and petro products (road oil, etc.) on their outsides, they are of special formulation to resist both brake fluid and petro products.
I *think* brake lines are multi-layered, with different formulations of rubber for the inside vs outside. The inside only has to resist brake fluid, the outside has to resist oil, salt, UV rays, and spilled/leaking brake fluid.
Bottom Line:
Never EVER apply petro products to rubber INSIDE a brake system.
ALWAYS check before applying DOT 3 or 4 to any non-brake system rubber.