Alright, I will summarize everything that has been said about this unit and add my two cents worth.
I am actually the owner of the shop that Jim used to work in and have been advising him on this question for the last couple of messages he sent to you.
Here is what has actually happened.
The hydros will wear on all parts as it is used. They are built to very exacting specifications. If there is even the slightest wear on the internal parts, the oil will tend to bypass these parts and leak the pressure. Jim did get one part wrong in his answer. There is very little chance that there is a part broken inside the hydro. They usually do not break parts. But even the slightest amount of wear will cause them to malfunction. In order to repair hydros it is imperative that you replace ALL of the parts inside the case that has any contact with the oil. This means the pump motor parts...etc. If any of these parts are not replaced, they will cause the pump to leak internally because of the wear and it will not move as it is supposed to. Hydro parts are EXTREMELY expensive and it usually costs nearly as much for the parts to repair them as it does to replace the entire hydro. I have been to many hydro repair schools and even after we have spent time tearing down the units so we can see how they work, the instructor then says to basically forget everything we have just learned and just replace the hydro. Most of the less expensive hydros do not even have parts available for them.
The only thing that I see that you can do is to drain the fluid out and clean the inside, and then install new fluid. When these hydros start to go bad, the slipping that they do creates a lot of HEAT and it tends to burn the oil inside the hydro. It then thickens to the point that it does not flow well through the system. If the oil is changed, you might have a chance that it would work a little better for a short while.
I do NOT recommend that you try to sell this unit to someone else UNLESS you disclose all of the information that we have talked about here. As you and James said, that would not be the right thing to do. But you will find that there are many people that think they can fix anything and will want to try. But if they know that they may have to replace the hydro before they buy it, then you have done your job and can still sleep at night.
Many of these hydros can go bad also if the customer washes his machine with water or leaves it outside. The water will be drawn into the hydro through the seals and contaminate the oil and cause it to wear out prematurely. Most people and even many dealerships do not really know the DANGERS of washing mower with water. In fact many dealerships are still washing them with a pressure washer before they try to repair them for customers. Washing mower is the second leading cause of catastrophic Lawnmower failure.
If you do not want to spend the money to replace the hydro, then after you do some of the simple things I recommended, the I would suggest that you sell it as described above. Remember that even replacing the hydro would be much less expensive than replacing the entire mower.
I hope that I have cleared things up a little.
As James said, please do not shoot the messenger as I am just trying to help you. Even if it turns out that you can get it working, what we have given you is the information that applies to most of these units when the act as yours does.
Keep me informed as to what you find as I would be very interested to see if you find anything other than what I have suggested.
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