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Replacing the proportioning valve isn't a huge job but it does come with some potential dangers. Since you're working with the brake hydraulic system if something goes wrong and there is a leak then you could end up losing your brakes suddenly.
Unfortunately there aren't any instructions in my service information for replacing this proportioning valve but I see that it only calls for a half hour of labor to replace it so it won't be very involved. The lines will all need to be removed from it and then you can remove the bolts that hold it. It doesn't appear that removing anything to get to it is necessary, but if the air intake is in the way and you would feel better without it there then you can certainly remove it.
It's possible that the lines could be seized and may twist off when removing them. If that's the case then new lines would have to be fabricated and installed.
When the new valve is in place then all four wheels will need to be bled.
If you decided it wasn't something you would want to repair yourself then any shop should be able to do the repair. You shouldn't pay more than about an hour of labor to replace it at whatever the shop's hourly labor rate is.
If it is leaking it's not a good idea to just keep topping off the fluid and not replacing the valve. The leak could become severe, possibly suddenly when you're trying to stop, and the brake pedal would go to the floor.
The emergency brake problem could be cable or shoe adjustment. If you have rear disc brakes there are emergency brake shoes inside the hat in the center of the rotor. The shoes may be damaged or simply out of adjustment. They will need to be looked at first and adjusted if they are ok, and then adjust the cables if necessary.
If you have rear drum brakes the emergency brake uses the same shoes as the service brakes. Again you'd need to start by adjusting the brakes and then move onto the cables if necessary.