Brake fluid gasket always ripping up, Why?

I have changed the break fluid Gasket, where you put in the fluid, 3 times in the last six months. Why is it ripping up?|||your either adding the wrong brake fluid or someone else did.contaminated brake fluid will turn rubber seals soft and mushy.if this is the case take to a mechanic asap.other brake componets may be affected.its a saftey issue|||it is the type of break fluid you useing|||What are you doing, prying off the cap???