Chances are good that it is simply the checkvalve on the vaccuum ball.
It is the rubber fitting where the engine vaccuum line attaches to the ball. You can usually get them at the auto parts store in the HELP section. You can add an external checkvalve and leave the bad one in place, if necessary.
It is common for that particular vaccuum line to get rubbed through or cracked. Same goes for the heater control valve line. If you have a heater control valve, it will be located on the heater hose near the firewall. Your leak could be there too.
2000 2.5 MO5D 4.10 one-wheel drive DD - gave to my older son
1990 STX 87 2.9 going in.....Junked it
2000 3.0 4WD M05D 4.10 S-cab Off-Road package with 235-75-15" tires, hotrod DD, lol