I'm leaking brake fluid in my the rear my truck

this is a front wheel drive car there fore there is no rear end like a rear wheel drive. the fluid leak at the wheels is probably brake wheel cylinder/caliper or brake flex hose. check brake fluid level if its down then check for brake fluid leak if not then you will have to check the rear area for some other fluid leak.

The leak could be due to a leaking brake line in the rear or a leaking brake cylinder. Have the car inspected for the leak by a reputable garage. The car will need to be towed, since it has no brakes.
If you do your own work, refill the master cylinder. Have someone depress the brake peddle while you look under the car for the leak.

If you have no evidence of leaking outside the drum, most likely your brake piston is leaking or the drive shaft seal is leaking. Pull off the drum and look for the piston which spreads the two brake pads normally situated at the top. Inspect for leaks. If the fluid is thin, it is most likely brake fluid. If it is heavier like oil and really stinks, it is gear oil from a leaking seal.

does the truck have rear drum brakes on the rear? if so then the fluid is leaking out of the rear wheel cylinders. or the master cylinder is leaking fluid into the brake booster.

usually an indicator of either a loss of pressure due to low fluid and/or air in the system (brake fluid leak) or extremely worn rear brakes (if drum brakes in rear). possible causes include master cylinder (internal fluid pressure leak), external fluid leaks (calipers, lines, wheel cylinders), rear drum brakes extremely worn down or severely out of adjustment. normally worn disc brakes may reduce pedal height SOME, but seldom "to floor". but the "mushy" quote combined with the "floor" quote suggests a hydrolic fluid pressure problem.

The leaking could be from your brake line, can you tell what color the fluid is and the texture? is is slippery or feels more like water? If it is brake fluid leaking that means you have a broke or busted line or a caliper/brake cylinder leaking which needs to be fixed quickly.

remove the rear tires and the brake drums and check and see if the wheel cylinders are leaking

Sounds like you have a leak in the brake system. The fluid you are seeing is brake fluid.

You need to get this fixed asap.

leaking fluid will not cause a brake to lock up unless the wheel cylinder is leaking and caused the brake shoe to swell. If so, you need a new wheel cylinder.

if your car has rear drum then the wheel cylinders could be leaking and in need of replacement. If it has rear disc brakes then it could be leaking fluid where the brake hose goes into the caliper or the brake hose my be worn or have a hole in it. Or it could be a a brake line. You should have it checked because you don't want air getting into the braking system.