How can I fix the 3 point linkage that drops on my IH 684 Tractor?

The TPL will not remain in any elevated position, without the lever being held. I suspect air is getting into the system somewhere as the brakes also loose pressure. Other wise it works well. Are there 'O' rings elsewhere, other than under the seat that might be faulty? If so, are they readily accessible? There is plenty of oil in system.