Hi and welcome Thank you for allowing me to assist you with your question today.
Do you mean when you release park brake it dumps 50 lbs? You may call it hand brake but I want to be sure.
What pressure does it build to? 120 psi is when air dryer should purge.
How long is slow?
Hold rpm around 1000 rpm start with no air in tanks, how long does it take to build up to top pressure?
Does air dryer purge?
What is top pressure it will build to?
While you are doing this use a bottle of soapy water and spray air compressor discharge line coming of top of compressor (steel braided line or tube) all the way to air drier to see if it is leaking.
Its a common issue since you can't hear it leaking over engine noise and once engine is off it stops leaking.This is a very common cause of slow air build up.
Let me know