Pembrolizumab in Treating Patients With Recurrent Glioblastoma

Inclusion Criteria: * Be willing and able to provide written informed consent/assent for the trial * Have histologically confirmed World Health Organization grade IV malignant glioma (glioblastoma or gliosarcoma); participants will be eligible if the original histology was low-grade glioma and a subsequent histological diagnosis of glioblastoma or variants is made * Patients must be at first or second relapse and clinically require reoperation for tumor progression within 4 to 6 weeks; Note: relapse is defined as progression following initial therapy (i.e., radiation, chemotherapy, or radiation + chemotherapy); if the participant had a surgical resection for relapsed disease and no anti-tumor therapy instituted for up to 12 weeks, this is considered one relapse; for participants who had prior therapy for a low grade glioma, the surgical diagnosis of a high grade glioma will be considered first relapse * Have measurable disease consisting of a minimal volume of 1 cm\^3 * Have provided tissue from an archival tissue sample or newly obtained core or excisional biopsy of a tumor lesion * Have a performance status of \>= 60 on the Karnofsky performance scale (KPS) * Stable dose of steroids for 5 days, no more than 2 mg dexamethasone (or equivalent) total per day * Absolute neutrophil count (ANC) \>= 1,500/mcL (performed within 14 days prior to registration) * Platelets \>=100,000/mcL (performed within 14 days prior to registration) * Hemoglobin \>= 9 g/dL or \>= 5.6 mmol/L (perfo…