Fuckin Radiation Mang.

My understanding is that it had been decades since purple man died in the suit and was sealed in the hidden room, and by the time the onwer of Fazbear Fright track it down and brought it back, the body had most likely rotted to the bone and/or having most of the soft tissue withered and mummified, no longer able to leak fluid or produce gas. It might be the cross beams and other animatronic components that held the corpse up in place.