E-brake light coming on continuously as well as e-brake not working well?

just a thought, but you are probably low on brake fluid or the fluid level sensor is bad.

I was thinking that the e-brake sensor is known to get loose (i have to tighten mine up)... And you probably need new e-brake cables....

The light on the dash does two things. It indicates when your parking brake is set. There is a switch on the handle. when it's up, it's on.
The other thing it does is let you know when you are low on fluid. If it is coming on when you are slowing down, it means the fulid is sloshing to the front of the Master cylinder, and causing the light to come on. IF you ignore it for a little while longer, it will be on all the time as the fluid will be low enough. Rather than just top it off, though, You probably need to look at your brakes. All the fluid that WAS in your master cylinder is now in your calipers because the brake pads are worn down. Time for new ones. The fluid will "magically" re-appear when you push the pistons back into the calipers....
Oh, and your Parking brake needs adjusting.