I have JB industries dv-85n
It pumps vacuum fine but when I shut it off within few seconds it starts sucking air back through the handle, anybody knows what to look for ?
broken check valve ( one way valve broken ). or it doesn’t have one. maybe install a vacuum check valve in line, or a gas ball valve. Does your pump have a manual valve on it?
Install a seperate one-way valve, also called “anti suckback valve”. I even had opportunities with old pumps that, when they shut down, they start running backwards. A non-return-valve will prevent this. Many pumps have one, but some cheaper ones not.