The invention discloses a relay piston assembly. The relay piston assembly includes a primary piston and a secondary piston, the primary piston includes a first piston body and a first piston ring portion disposed around the first piston body, and the secondary piston includes a second piston body and a second piston ring portion disposed around the second piston body. A spring base is disposed atthe outer side of the upper portion of the second piston body, an adjustment spring is disposed on the spring base, and an adjustment member is arranged in the second piston body and used for adjusting the nesting position of the spring base outside the second piston body so as to change the initial elongation of the adjustment spring. The invention also discloses a foot operated brake valve capable of adjusting the output air pressure difference, and the foot operated brake valve includes an upper outer shell, a lower outer shell connected to each other and the relay piston assembly arrangedbetween the upper outer shell and the lower outer shell. The output air pressure difference between the two circuits of the foot operated brake valve can be adjusted to meet the requirements of different customers for different pressure differences.