2004 GMC Sierra Cranks but will not start Brake booster light

[blundy74]

My 2004 GMC Sierra 1500 5.3L cranks (turns over) but will not start and the “service brake booster” light is visible. The strange thing is sometimes it will not turn-over at all and turning the key does nothing.. Further confusing the issue, sometimes it cranks and runs perfectly with no signs of problems. I've used a diagnostic code reader and no codes are present.

 

Does anyone have an idea what could be causing this issue? Any help is much appreciated!