A piston pump assembly includes a first pump having a first cylinder member inserted in a piston hole of a pump housing, and a first piston inserted in the first cylinder member, and a second pump having a second cylinder member inserted in the piston hole, and an annular second piston slidably fitted in an annular groove formed in the outer periphery of the first piston, and also axially slidably inserted in the second cylinder member. The first cylinder member axially presses the second cylinder member, thereby preventing the second cylinder member from being dropped out of the piston hole. The second cylinder member has an annular seal surface in contact with a shoulder surface formed on the inner periphery of the piston hole. Strain-absorbing protrusions are formed on the end of the second cylinder member in abutment with the first cylinder member. The protrusions are compressed when they are pressed by the first cylinder member.