Front axle pivot (oscillaton) - The bushes in the pivot wear out if not greased (like everything). It is a fairly big job to do, as the pin is usually siezed in the hole.
Engine smoke - It is quite common for these to blow a little blue smoke, not because they are worn out, but because they haven't been worked hard enough. A good workout at max power generally solves this.
Electrics - The starter and alternator is generally bullet proof. Gauges aren't of the highest quality, check they all function. The tractors don't have any electronics that are needed for them to function, as long as it starts and charges.
Clutch - The clutches are normally pretty good, the 5011 only has a linkage operated clutch, which don't give any trouble. The buttress caps in the clutch can wear out and break, but this can be fixed without splitting them. The PTO clutch generally doesn't give any issues, other than the cable for it siezes if not lubricated.
Gearbox - They have an issue with the selector, which can cause two gears to be engaged at the same time. You can tell if this has happened, the tractor will be locked solid if the gear stick is in gear, and it will reverse when in neutral. It takes an hour to fix (no parts), and there is a modification to stop this happening. The levers for the high/low range and PTO have a roll pin to retain them, the pin has a habit of breaking at the most inconvenient time if the lever is forced too hard, then the lever will be useless until the little pin is replaced.
Hydraulics - The hyd pump in them is crap, and wears out in no time if used for loader work etc. They are resonably priced and take an hour to replace though. Take a pressure gauge with you that connects to the remote coupling, the pressure should be around 2400psi from memory. I have replaced my pump with a crankshaft driven pump now, much better, and fast hydraulics.
3 Point linkage - The Zetormatic system works great, and generally gives no trouble, other than the lift piston seals are of poor quality, causing the hitch to drift down. I have replaced the seals in mine with proper
hydraulic piston seals, and it will now hold a 1.5T rototiller up for days if needed. It is not a real big job to replace them, just the hitch lift cover is VERY heavy.
Brakes - The Master and slave cylinders were of poor quality when manufactured, though they have now been fixed. They are cheap to buy, buy the whole cylinder if needed, don't replace just the seals, as the cylinder is the poor quality part. The brake slave cylinders take a bit of work to replace, as the whole final drive needs to be removed. Very easy with an engine hoist though. The band type hand brake works great, only the cable for it siezes like the PTO cable if not lubricated.
The above may seem daunting, but these problems generally only will happen once in the life of the tractor, and they are all very cheap and easy to fix. Most problems come from poor maintenance. Keep the oils, coolant, filters and brake fluid fresh, and everything greased, and you will be unlucky to have a problem with them. They are a great tractor, the you can safely squeeze another 10% more HP out of the engines, and they have the weight to
help get them horses to the ground, which is what counts.
P.S. we at Zetorworld do answer everyone's posts, we just take a little longer, as we are normally out on the tractor working the fields, not behind the computer looking how to fix them