Diaphragm valve with internal leak detection and improved external leak sealing performance. In diaphragm valves comprising diaphragms that have shields and backings, a rigid seal plate interposed between the shield and backing prevents peripheral regions of the shield from cold-flowing into the backing, despite the compression forces produced by tightening the fasteners that secure the diaphragm to the valve's bonnet assembly and body. An access port passing through the seal plate provides an interface between a void located between the shield and the backing and a breach detector. The breach detector is configured to detect pressure, volume, moisture or mass deviations in fluid disposed in the void, and the deviations indicate whether there's a rupture in the shield or the backing. The breach detector can be connected to an alarming device or signaling system to provide warnings and alerts to valve operators, process control panels or data communication networks.