Okay.
The vacuum is picked up from the intake manifold and sent over to a solenoid switch on the firewall or the actuator (early models).
When you pull the pto switch on the dash it energizes the solenoid and sends the vacuum over to the actuator which causes the arm to move and tighten the belts.
Make sure all your hoses are on.
Pull the hose off of the solenoid that goes down to the actuator (not the one going to the engine) and suck on it. That should cause the arm to move on the actuator and tighten the belts.
You could have a stuck solenoid or leaking actuator causing your issue. Try the above and report back. We'll get you slinging grass in no time.
