Well, I assume it was your Blazer. That kills one possibility.
I had the exact same thing happen to me.
had the master cylinder replaced on a Friday under warranty.
Drove fine going home, and to the hunt Saturday.
About 9 o'clock, I pulled up behind the bunch in the woods, and hit the brakes.
Full on plum
My old Jeep had a single circuit brake system, and lost brakes regularly. Like a couple times a week when I was making roads with it during hunting season.
So, I have a very well honed lost brake reaction.
I dropped it into low, a quick look told me that the E-brake would not stop me in time, so I picked out the best trees, and took them out.
If you have seen the pics of my front bumper, thats what its for.
The local dealer in the area was open on Saturday, so I headed that way slowly using my emergency brake.
I kept pumping on the way, and they got better.
When I got there, I told them I wanted a new master cylinder. Like you, I had no fluid loss, and no visible leaks.
They looked in and saw it was a new one. They checked and told me that apparently the other shop had not bled it properly.
They assured me that they could take it off, bleed it, and it would be fine.
I agreed with them. I told them I had all the confidence in the world if that master cylinder was taken off and bled, it would give long and faithful service.
Just not on my truck.
I knew all these guys, had hunted with some, and had had most of them over to my parties.
They just grinned and put me on a new one.
Never had anymore trouble.
If you can't find anything else, my vote is for the MC. (Not now, keep reading for my better theory)
I was once riding with a guy who had so much run-out on a disk due to a wheel-bearing problem, that it would drive the caliper pistons back.
First press of the pedal would go almost to the floor while it pushed the pistons back.
Next press had good brakes.
Of course, not long after he mentioned the strange brake problem, the left front wheel came off, which relegated the brake anomaly to an afterthought.
If your vehicle was newer, here is a problem that I post from time to time in automotive forums when brake problems get mentioned.
Surprisingly few people know about this one.
Up to a certain year, split brake systems had the front brakes on one circuit and the rear on the other.
Then, someone had a bright idea.
Most car companies started doing a cross split. Where one circuit uses the left front and right rear. And the other circuit uses the other two wheels.
Sounds like a great idea, and was. Except for one thing no one thought of.
Suddenly, there started to show up a small but persistent number of brake failures that were almost impossible.
You would have a total failure of the brakes. But, after the accident, or after they got stopped, the brakes would be working fine.
Several people were accused of driving under the influence, reckless driving, whatever, and they all loudly screamed that their brakes failed.
But the system seemed fine.
Finally one engineer got called in to evaluate enough of these failures to see a pattern. Even though he had testified that the brakes were fine.
All the cars showed signs of overheating in the rear brakes. Most cases were unusual, but not serious enough to cause a problem, and all of them worked fine.
Finally someone put all the pieces together.
It always happened after the car had been driven for a while, had stopped for a short while and then started off only to have no brakes the next time they tried to stop.
You know, this is sounding more and more like your problem.......
Anyway, the cause was the emergency brake had gotten left on. This caused the rear brake drums or rotors to get extremely hot.
When the car stopped, that heat got transferred into the brake fluid which boiled.
Normally that steam bubble would have only killed the rear brakes.
In this case since the rear brakes were in both, it killed both circuits.
After the brakes cooled, the brake fluid steam condensed and all was well.
You know, if you have rear drums, and they are not adjusted up, then most all your braking is going to be on the front.
Not only is this going to cause them to get hot, but if you were to lose the front, odds are you would lose them all because the rears would take all the fluid the master could send and the shoes would not reach the drums.
I'm thinking moisture in the fluid lowering its boiling point, hot rotors when you stopped for the ranger, heat soaking into the fluid while you are holding the brakes with the pads pressed against the hot disks.
When you release the pedal, the sudden pressure drop caused a flash boil in the fluid.
Gas in the line, no brakes.
Think back about what happened and when, and see if thats possible.
Luckily i shoved her in low and limped it to my grandmas house nearby.
