I replaced my rear brakes. Does the emergency brake automatically adjust itself?

I'm assuming you've been replacing the brake shoes in the hand brake. Rear drum brakes are designed to automatically adjust for slack as they wear. You will have to wind back the adjusters in both rear drums and the threaded adjuster on the lever end of the cable. This should give you enough slack to reconnect it.
**Chock the wheels or put the vehicle on axle stands BEFORE you do this.**
Wind the lever end adjuster back to approximately half way. Pull the lever to the correct 'On' position. Rewind BOTH rear drum adjusters back into a position where they grip the drum firmly but spins freely when the hand brake is released. With the hand brake off spin the drum. You should be able to hear a very faint noise as the drum skims the shoes.

Depends on the type of vehicle. Some have adjustments on the cables under the vehicle. Others have an adjustment that is automatically done when you back up and aggressively push the brake pedal.

could be that emergency is stuck on, sometimes pulling of the cables will loosen them up. If you are trying to adjust the brakes, are you sure you are backing them off and not just tightening them more? Sometimes a bad rubber hose can break down and maintain the pressure on the brake cylinders, locking them up. I would remove the wheels and back off the adjusters and free up the emergency to get the drums off. Once brake adjustment problems are eliminated to can work at figuring out if the rearend itself is broken/locked up.

I believe you have drum brakes on the back. Not sure. If so, it probably is the automatic adjuster is frozen. This comes into play when you go backwards and apply the brakes. It is used so that the emergency brake is in proper alignment, that is, it only takes a few clicks before the emergency brakes start being applied. If there were no adjuster, as the brakes go old, you would lose the emergency brakes entirely.

Over here we call it a parking brake - hardly any of our cars are autos.

Parking brakes usually connect to the rear disc brake calipers but some manufacturers use a small drum brake in the disc centre for parking purposes due to the complexity of the caliper with integral parking brake.

When adjusting a parking brake it is important in most designs for the cable to be adjusted only to remove any free play without moving the caliper levers from the backstops and then if the parking brake lever or pedal travel is excessive it must be because of wear or other faults. Sometimes that basic requirement must be taken with a pinch or two of "experienced" salt...

Ford have a self adjusting parking brake I have experience of. A spring within the system preloads the cable eliminating all slack in the cable so the cable moves the caliper levers from the first. The main problem with the system is the preload spring is sometimes strong enough to overcome the resistance of the caliper spring and cause a binding brake. No manual adjustment was provided.
In that case I find fabricating and fitting brackets to anchor additional return springs to the calipers is a must.

you may have a bad ABS module but you may need further diagnostics to confirm this

FIRST adjust rear brake starwheel while spinning tire FOWARD. then alongside drivers side of car find emergency brake cable adjustment (long threaded rod with nut) . hold one end of rod with vise-grips and turn nut until emer. brake cable has less slack in it. now go inside car and apply e-brake.you dont want the pedal to stop right away but u also dont want too much"travel" in the pedal.so about halfway is IDEAL. IMPORTANT- once the brake cable is adjusted and the car wont roll foward in gear with the brake on and applying a little "gas"-make sure with brake OFF that rear wheels turn freely. hope this helps.

The adjustment is under the brake handle inside the center console.

BUT.....most of the time the emergency brake is loose is because the brakes are worn down or the automatic adjusters aren't working.

So before you go to all of the trouble of adjusting the emergency brake you may want to check the wear on the rear brake pads.

first you need to check the rear brakes ,the emergency brakes work off the rear brakes, there is an adjustment on the center of cable that goes between both rear brakes,hope this is some help,thanks robert b.

If the whistling changes when applying brakes it is just your warning indicator letting you know that the pads are getting very thin.
You did not give a more detailed description so I hope this helps.
Sam