621 B loader brake light

I have a 621 B loader and the brake light is on until I rev it up and it quickly goes off when RPM lowers. Any thought on what it could be? I have a gauge on the regulator at idle it is about 1000 PSI when reving up it goes around 2000. But im not sure if I am even using the right port. Does anyone have a troubleshooting for brake light or a diagram of regulator and brake system?

Thank you for any help

Sounds like you need to adjust your cut in and cut out pressures. Your brake pump may also be worn out, which is probably why you get a higher pressure reading at a higher engine RPM.
Try adjusting your brake valve first, (I would recommend counting how far you adjust the screw, so you can set it back if it doesn't make any changes)
I'm assuming you don't have a flow meter, so there is no real way to properly test your brake pump itself, but the brake pump is likely worn out based on the age of the machine.

Check your accumulator pressure also, they tend to leak down over the years.