Pull the master cylinder off the booster just a little bit, you do not have to take the brake lines loose. Look behind the master cylinder, see if it's wet with brake fluid. If it is, the master cylinder has leaked into the booster and ruined the booster.
I'll check when I get a chance, but I'm about 90% certain that this is not the case. The mastercylinder was a new unit installed about a week before the booster back in 2018/19. Not a lot of miles on it since. Not that new parts can't be bad or fail in short order, but the reservior has not shown any signs of leaking or loosing fluid.
+1 ^^^^
This type of power brake system works using outside air pressure
The vacuum in the booster is lower air pressure than outside air pressure
In the cab where the brake pedal push rod is in the booster is the Atmosphere Valve
When you press brake pedal the push rod pushes open the atmosphere valve allowing outside air pressure in and this is what assists you in pressing down the brake pedal
Diagram here:
https://www.researchgate.net/profile/Liang-Li-21/publication/326618599/figure/fig3/AS:662375493550080@1534934032966/Diagram-of-vacuum-booster-system.png
Start engine to build up vacuum in booster, shut off engine after 30seconds, doesn't take long, booster stores enough vacuum for 3 or 4 pedal presses, in case engine stalls while driving driver will have enough vacuum to safely pull over still having power assist
Get down low under the dash and press brake pedal, you should HEAR air being sucked in
If not the atmosphere valve is bad, so no assist
You might be onto something there. Did not really know how the insides of the booster worked. I'm sure I've seen it in years past, but that diagram drove it home. Vacuum goes to both sides of the diaphram until pedal is pressed at which point the back side is opened to atmosphere and vacuum provides braking assist. If when I'm pressing the pedal it doesn't open to atmosphere then I get no assist. That makes sense for what is happening.
Power assist doesn't work at all from the time I start the truck. Truck needs to be tuned, but I'm getting decent vacuum to the booster. Put a gauge in line between the engine and booster and it is showing 18-20 inches. The vacuum check valve flows like its supposed to. There is definitely vacuum building in the booster considering that it sucks in air when the check valve is removed.
The way the booster is mounted on these old trucks it is not bolted directly to the firewall. It actrually has standoff brackets that keep all of the booster outside of the cab with a long pushrod that goes through the firewall to the brake pedal. With the truck running I doubt that the vacuum would even be audible under the hood, but between seeing how it works and reading what you wrote, I'm fairly certain that this atmospheric valve isn't working. This isn;t my setup, but an aftermarket kit for my truck with similar mounting style to stock.
Sounds like regardless of what is causing the problem, the solution is going to be replacing the booster. Guess I need to decide if I'm trying another
reman booster (54-73112), or getting a new modern style unit from
Battle Born Brakes. The one I have now is a Cardone reman and I think most of the big chain store remans (Wearever, BrakeBest, Duralast) are probably the same booster stuck in a different box. The BattleBorn unit is allegedly new manufacture made for the application.
I'm not returning the core I have. When I was doing the booster upgrade boosters for these trucks were hard to find. I could have sent my booster off for rebuild if I had one, but I didn't. I'll be saving the booster I remove so that if I encounter the issue again I can ship it off for rebuild, so eating the core charge. With that in mind, there is about a $!00 difference in price, $150 for the reman and $250 for the new unit.