The sponginess problem with silicone-based (DOT 5) fluid comes from its higher solubility of air. When the fluid heats up, the air comes out of solution, making the pedal softer. When using silicone fluid, it's always a good idea to cook it to near its boiling point for a period of time, then be very gentle when handling it.
Even when fluid expands, as long as the linkage is set up correctly and the return port is open, the brakes shouldn't spontaneously lock up.