A relief valve has a valve body, a plunger assembly coupled to the valve body for selectively opening and closing the relief valve, and a support member secured to the valve body. A lockout lever is against the support member, forming a vertex. A spring rod is coupled to the lockout lever, and a spring is coupled to the spring rod, providing a bias force against the plunger assembly to normally close the relief valve. In a lockout position, the lockout lever is moved to pivot on the vertex against the support member to further compress the spring, which increases the bias force and, in turn, a cracking pressure of the relief valve.