Engine bleed air system comprising a backflow prevention valve

A valve assembly (300) includes an outer housing (301) having a first opening (305) in fluid communication with a first fluid, a second opening (306) in fluid communication with a second fluid, a third opening (307) in fluid communication with a third fluid and a fourth opening (308) in fluid communication with a fourth fluid. The valve assembly (300) includes a piston (304) configured to slide within the outer housing (301) into a first position at which the third opening (307) is blocked from fluid communication with the fourth opening (308) based on a fluid pressure of the first fluid being greater than a fluid pressure of the fourth fluid. The piston (304) is further configured to slide within the outer housing (301) into a second position at which the third opening (307) is in fluid communication with the fourth opening (308) based on the fluid pressure of the second fluid being greater than the fluid pressure of the fourth fluid. Also disclosed is an engine bleed air assembly incorporating the valve assembly.