Abstract: Apparatus for protecting a fluid pressure system from over-pressure, comprising a dome-shaped, reverse acting rupture disk adapted to be installed in an outlet of the fluid pressure system with its convex face resisting the system pressure, the reverse acting rupture disk being adapted to abruptly snap over center when subjected to a pre-determined pressure level, and a knife blade assembly disposed adjacent the back or concave face of the reverse acting rupture disk for cutting it as it snaps over center. The apparatus further includes a dome-shaped, prebulged rupture disk separated from the reverse acting rupture disk by a spacer ring, the prebulged disk having its concave face toward the reverse acting rupture disk whereby the prebulged disk entraps any fluid that may leak past the reverse acting rupture disk and will reliably burst when its concave face is subjected to the system fluid pressure upon the rupture of the reverse acting rupture disk.