The rear brake cylinder is leaking.
You will need to have the brake cylinder replaced. I recommend replacing both sides so that brake pressures will be equal. Once replaced, its a good idea to flush the brake system with fresh brake fluid. I suggest having a qualified technician handle the repairs to ensure the brake system is serviced properly.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing