...brake pedal feeling. No leaks!? Can it be the brake master cylender or maybe the abs? The front brakes work fine but the rear brakes dont brake at all.
If you put other than brake fluid in the master your brake system is contaminated and everything must be overhauled. No if ands or buts. Anything other than brake fluid in there will swell up all the rubbers and your brakes just will not work. Feel the brake fluid, rub your thumb and first finger together.If it feels greasy (very slippery) might be contaminated . If your finger and thumb doesn't slip very well it should be OK. You could also take a sample in a clear glass jar and see if it has a foreign fluid in it.
You might have a proportioning valve stuck in the lines going to the back of the vehicle .