It has been shown that even very simple cubic equations of state like the Redlich-Kwong one-fluid model can reproduce the closed-loop liquid-liquid immiscibility of binary fluids. The global phase diagram of Redlich-Kwong model has been calculated in the region of closed-loops. For the first time, two boundary states, namely, the critical pressure step point (CPSP) and the critical pressure level point (CPLP) were calculated in the global phase diagram. The second CPSP curve was discovered that sets a limit on the existence of closed-loop phenomena. Thermodynamic description of a CPLP was also obtained for the first time in the present paper.