The ornamental design for a toiletflushvalvebase, as shown and described.
FIG. 1 is a perspective view of a toilet flush valve base showing my new design;
FIG. 2 is a perspective view of a toilet flush valve base showing my new design with the float shown in phantom lines;
FIG. 3 is a top view of my new design illustrated in FIG. 1;
FIG. 4 is a bottom view of my new design illustrated in FIG. 1;
FIG. 5 is a front view of FIG. 4;
FIG. 6 is a left side view of FIG. 4;
FIG. 7 is a rear view of FIG. 4; and,
FIG. 8 is a right side view of FIG. 7.
The broken lines in the figures is included for the purpose of illustrating potions of the valve in FIGS. 1, 2 and 3 and to show environment in FIG. 2 and form no part of the claimed design.