boom swing problem

Hello there, Can anyone please help troubleshoot boom swing function on mini excavator? Right now the boom wants to swing to the right, all the time! The boom is already as far right as it goes yet when I turn the machine on, bring down operation lock lever, it's still trying to swing the boom to the right ( I can sense the pump trying to push the swing piston). The swing is controlled by a foot pedal. If I press and hold foot pedal to swing left, the boom will slowly swing left. But as soon as I let go of the foot pedal the boom swings right, it doesn't stay put as it normally should. It was working fine until the other day. Machine is a Thomas 75 excavator. I'm still learning about hydraulics, any advice you can offer would be appreciated. Thanks. -- pebbles.