Totally Baffled: loss brakes on 1987 pickup

The truck is a 4X4 pickup with a 22RE engine, and an automatic transmission.
A few weeks ago I noticed very infrequently that my brake pedal kept slowly sinking.

This only happened sometimes, while I was waiting for the traffic light to change.
The brakes always stopped perfectly and this symptom soon disappeared.
Today after a month past by, it happened again and I totally loss all brakes.
If a pumped the pedal 5 – 7 times, it briefly gets pressure again but disappears again almost immediately.
Luckily I was near the house, and inched my way back home with massive brake pedal pumping.

It seemed a little like there is air in the lines, but I’ve never seen anything this bad in any other vehicle.
I can find no brake leaks at all and the brake fluid level is full to the top.
This past Spring I had preformed a full brake system overhaul, which included the following:

Every entire steel brake lines, all rubber brake hoses, master cylinder, brake proportioning valve,
Rotors, calipers, drums and shoes (only thing left original was the vacuum booster.

I am totally stumped – re-bleeding the master cylinder and all the wheels has no improvement.
The front brakes are just as bad as the back brakes (basically NOTHING).
The parking brake will still engage, but nothing at all from the hydraulics…

Has this ever happened to anyone at this forum?
Can anybody suggest any kind of help?

Help Please…

OK, if your symptoms are this -

You apply pressure to the brake pedal and you have some brake action, then the pedal sinks to the floor under continued pressure, the brake Master Cylinder may have an internal leak. Pumping the pedal and regaining some braking pressure would be normal for this condition.


Also, if the back drum brakes are not properly adjusted, getting full proper braking action will be difficult.