Back pressure when bleeding brakes

[Stepper86]

Something wierd happend today when I was bleeding my brakes.

I have rebuilt everything in the system except for the master cylinder that was put in just before I bought the truck.
So I had a friend pushing the brake while I went to loosen the bleeder, nothing happens, so I figured that there must be some gunk in the bleeder.
I ask my friend in the truck to get off the pedal while I remove and clean out the bleeder. When I unscrewd it we hear a popping nice from the engine bay. I got up to see whats going on I found that the lid has popped off the fluid container on the master cylinder and that my engine bay now looks like a bomb went off. Brake fluid everywere!

I rushed to rinse everything off, then refilled a little and pumped the brakes to see what was going on. Nothing happend, so we continued to bleed and now the brakes work like they should.
At first I tought that it must be the master that has blown a seal, but it can´t be since it now works normaly, right?

Any ideas what might have caused this? I never had somthing like it happen before.