This randomized phase II trial is studying the side effects and how well giving bevacizumab together with irinotecan or temozolomide works in treating patients with recurrent or refractory glioblastoma multiforme or gliosarcoma. Monoclonal antibodies, such as bevacizumab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. Bevacizumab may also stop the growth of tumor cells by blocking blood flow to the tumor. Drugs used in chemotherapy, such as irinotecan and temozolomide, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving bevacizumab together with irinotecan or temozolomide may kill more tumor cells.