I had same issue, no brakes all of a sudden.
And at the same time another colour on the exhausts. Scared the hell out of me then, but I read about sucking fluid here, so I calmed down. Also had a leaking master cyl in the front of it.
So I just ordered a new master cyl from Alretta Truck Parts, and a factory rebuilt hydrovac, and followed the instruction Geoff fixed to remove all air.
Brakes are superb. Just waiting to blow a wheel cyl now....
