1995 Toyota Celica Brake Cylinder

I checked my rear (drum) brakes the other day. I noticed the wheel cylinder was leaking so I purchased a new one. After installing the new cylinder I tried to bleed the brakes. When I applied pressure to the brake pedal the left side of the Cylinder popped right out and it started leaking again. I am wondering what could be causing this to happen and how I can fix it. Hopefully I can press the new Cylinder back together and re-use it but I'm wondering if it will just break again and what I can do to fix it.