Have had some success recently making leather thicker and stiffer for flapper valves.
Place a 3 inch square piece of leather in boiling water for about 30 seconds. Pour cold water over it until you are able to handle it. Flatten out the curled up leather and weight over night. (Not too heavy a weight) Cut flappers when dry.
This increased the thickness by about a third. The amount of boiling time will yield different results.