Hi: When I take my thumb off the throttle my brakes do not work as fast as they use to, it take awhile for them to engage. Is there something I can do to fix my Pride Rally 3 wheeled scooter. I think I got it in 1998, so it is 13 years old and still works great except for the brakes. When I am driving it, it is no problem, but when I take out of trailer is is scary taking it down the ramp when wont stop quick.

A 13 year old working brake on a scooter is remarkable. This brake part is available and can be purchased for between $60 and $260 retail depending on model and SN. Labor is under 1 hour. You can do it yourself, installing it on the end of the motor.
If you want to clean out the dust of the disc brake you can try a strong spray of air. The dust could cause slippage if the brake were a year or two old.
After 13 years you are ready for a new brake and a new scooter so I am not sure you want to put a lot a money into repairs.
The throttle potentiometer also controls the brake engagement when you release it. You may want to invest in a new throttle pot as well ($80). This item wears out regularly. Hope this helps.