M Press Fittings

Plan ahead
Take a couple of minutes to take a look around the area to make a plan on how to proceed with the top consideration being a plan B if all does not go well. Plumbing just seems to have the potential for pitfalls, so it makes sense to plan for what could go wrong. Especially with older plumbing sources, the right types of M press fittings can be hard to find and may take something other than just the big box types of stores. it is good to know what company has the specialty supply of parts you might need. If you tend to try and fix stuff yourself, make sure you have the tools ahead of time so you do not get jammed up some Sunday night or holiday when everything is closed. Have a bucket that will fit under the fitting to catch any water once it is opened, a couple of towels or rags for any spilled water and you are ready to start.

Shut off the water
The second part of planning ahead is knowing that things happen and to be prepared for the worst. It’s a clever idea to shut off the house water supply before you start the work that may exert a pretty good amount of force on the plumbing. Test and confirm you can shut the water off where it comes into the building. It is a valve that may not have been used in many years and may be worse than the problem you are trying to work on. So, just check because not being able to use the main shut-off after you have broken a plumbing part by mistake or with too much force may compound the problem more that you ever want to think about. If there is one frozen part, there may be more in the system.
Knowing that a M press fittings or part is stuck means there is going to be some degree of fight on your hands, but the ideas presented here should make it easier and hopefully prevent a bigger project. A job that has anything to do with older plumbing and water involved has the potential to go wrong and if it does go wrong, be ready to deal with the unexpected. X number of gallons per minute can be a lot of water flowing where you do not want it until you can run to a shut-off someplace else in the house. You can bet that run will be a stressful one! So, if you are working on plumbing, shut the water off at the main just in case. What do you have to lose? You may avoid a lot of sheetrock repair, wood flooring repair and hopefully not have to pump out a basement.

Do the project when supplies are available
Knowing what your plumbing supply house has and what the big box stores have can be important to know in advance. The key thing is to know the hours they are open in case you have to get the odd specialty part that you didn't expect. You certainly don't want to start something that is unknown just before most stores close and end up being without water for a prolonged period in the whole house or even a section of the house.

Physically and chemically clean the area
Take a couple of minutes and get any accumulated stuff out of the way in the affected area you are trying to loosen. A wire brush is great to clean up anything loose that has accumulated like rust or lime. Then, whatever remains can be dissolved pretty well with a product like CLR - Calcium Lime and Rust Remover. It also works great to take the green spots out a tub were a faucet has leaked for a while. If the white lime is heavy, it may may take some soaking and multiple applications.

Use the right tool with good contact
Transferring torque on a fastener, especially a stuck fastener, NEEDS to be done with the right tool. If possible, make sure to use two tools: one to move the part that has to be removed and the other held in the opposite direction to stabilize the rest of the plumbing so it does not get too much torque and break.
The more tool-to-fastener contact there is, the better chance of success. To grab a tool that is handy and make it work compared to the right tool that makes complete tight contact between the tool and the work can be a recipe for disaster. A good example is open-end box wrenches that grip as tight as possible and prevent the flexing that can happen with an adjustable wrench. It is like using vise grips when a wrench should be used or the wrong-sized Philip's head screwdriver that strips the head. The job just gets way harder.
Another good example is locking pliers are usually NOT a satisfactory solution since the locking pliers are much more likely to slip and create damage. The damage from the quick and easy try with the wrong tool. like vise grips, can crush or mangle the pipe. Locking wrenches that apply a lot of squeezing action to get a grip may be like "God's gift to the handyman," but can be the devil's work with plumbing fixtures and copper pipe.

Every action has an equal and opposite reaction
Bottom line: if you grab onto a pipe and start to pull on it hard, then you are pulling on whatever holds the pipe in place in the wall or floor. If the connection is rusted, the straps that hold down the pipe may also be weak and you may be placing a lot of leverage against a pipe that could take a lot of demolition to get at if it is a random break. So, balance the effort on one side of the connection with effort on the other side.

Apply firm, gentle pressure
There is an old saying to help you remember way to turn the part you want to remove from a connector. It is a little jingle that goes like this: "Righty tighty, lefty loosey." This can help you from fighting against yourself. Also remember the saying fits relative to the connector and where you are standing. I think that is being anthropocentric. Now that is a big word!
Your first attempt is really just an evaluation and assume it is a test. The first effort is to see if it will come apart with "normal" effort. If you are reading this, you may well already know you have a problem and are past this stage. Remember, a considered approach works way better than flooding a room. Judgment and a feel for problems only comes with experience. Even if the connection or nut you are taking apart looks bad, a lot of times it may come apart easier than you expect.