Yanmar Three Point Hitch Will Not Lower

If your Yanmar tractor's three point hitch will not lower then you could have a couple different causes. 

The solution can, sometimes, be very simple like the linkage from your control lever to the valve has come loose. Check all of the visible linkage to see if it looks like it is doing what it is supposed to. 

The problem might also be a sticking hydraulic valve or a tiny piece of debris or a piece of an o-ring has got stuck in a tiny port and is blocking the oil from leaving the cylinder.  To test this theory you can remove the lockout valve and see if the arms will go back down. You MUST BE SURE that there is no weight on the lift arms when you remove that valve or you will get a very high pressure blast of oil and the three point hitch can come crashing down. You will also lose all of the valve components inside. You might need to jack up the implement to get the weight off and slowly lower it down. 

If the test above does not lower the hitch then the problem is often times mechanical. If the lift piston cylinder has been allowed to rust (un-used for a long time in humid areas or got water into the oil through a leaky shift boot) then the lift piston can get wedged in that rusty cylinder and will be hard to get back down. Sometimes you can gently tap the cylinder arms down and get them free but other times you might need to disassemble and hone out

the cylinder to remove the rust. 

The problem might also be that the lift arm pivot bushings (located right behind the lift arms) have been corroded or rusted and are sticking.