International 9100i Air Brakes 30psi drop when brake applied

Most likely if chambers and all that are still stock, crud/water in the tank/s. Water reduces the available air volume so real light use looks like it dumps a ton of the pressure but is should also come back super quick.

If chambers or whatnot were replaced with different sizes then it could be a capacity issue as bigger needs more air.

 

You probably have an air leak when the brakes are applied , brake chamber , air line or valve. Air the truck up and turn it off. Have someone apply the brakes while you walk around and listen for the leak.

 

When was the last time you drained your air tanks?

Had a truck in the fleet one time with the same problem. Tanks were about half full of water.

 

If it drops down enough to set off the alarm and recovers within 1 second I'd say there's likely not an air supply issue. Its impossible for a compressor to recover that much pressure that fast in a standard size air tank.

You have all the air lines put back in the exact same place on the components you replaced?

I'd start by sticking a pressure gauge in the secondary tank and run it up to the cab, see if its response matches the dash gauge. If the dash gauge acts screwy and the test gauge acts normally, I'd focus in on the gauge/buzzer and their associated hoses/wiring.