2009 yamaha grizzly 550 brake issue

:cussing::cussing:
Was changing brake pads on the 550 the other day and after I put them in I pumped the lever several times to close the calipers and bring pressure up. Well I heard a pop and have no rear brakes. Funny thing is no fluid leak. Is this probably a caliper seal or master cylinder ???
Any input from those that have experienced similar is greatly appreciated
Thanx:gcrules: