I have replaced a couple of left rear wheel brake cylinders. I just had to replace a porportinal valve and when i went to bleed the system another left rear cylinder is in pieces in my drum. The chance that i could be doing something wrong is at the top of my list so any help would be great.

 

How are your brake shoes? If they are worn hard, they can allow too much travel in the wheel cylinder.

When you put everything back together, how much room is there between the shoes and the wheel cylinder?

 

Same thing if your drums were turned too much also If your shoes are not adjusted correctly and the self adjuster is not working, that could add to the problem.