No Brakes!

My truck is a 1992 Dodge Dakota. The front brake line burst! The rear brakes should have worked to stop the truck. They didn't. The pedal went to the floor. Brake fluid from both reservoirs not just the front shoots out the broken front line when the pedal is depressed. My mechanic says the part called the proportioning or combination valve could be the problem, but is no longer available. Any help greatly appreciated!
Author: Larry21