2nd cracked MC cap in 1 month!!!

[harrison sapir <hsapir@earthlink.net>]

The patient is my 89 90q. ABS, all that. So about three weeks ago, I
replaced a cracked master cylinder cap with a used one. I checked it out
thoroughly, and it appeared to be solid. Now tonight, the low brake
fluid light came on, and I find another cracked cap! What the heck? Now,
I will not have a problem replacing this one, but what could be causing
this problem? too much pressure in the system? How would I alleviate the
excess pressure? Am I just an idiot for using a used cap? Any input is
appreciated.

harrison