Most always when going down slope, we are not accelerating and vacuum reserve therefore is diminishing vs in town driving.
I'd recommend that everyone experiencing this fade problem do a vacuum test on their brake system as we are getting older now and all those hose joints have the potential to leak. You'll need a vacuum gauge and a small "T" fitting to do a test. Temporarily install the gauge in a small vacuum line under the hood and see if it can get up to 18" of vacuum (the standard). Once there, shut the engine off and see how long it will hold the vacuum of 18". It should hold it for at least 5 minutes, if it's falling you have a/some leaks. Another test is for the brake servo. Once you got the leaks discovered & repaired and it will hold vacuum, run the vacuum up to 18" again, shut the engine off, and use the brake peddle to do a "medium" braking pressure. The vacuum should drop about 6" immediately and while maintaining that same foot pressure NOT bleed down more than 2" more in the next 30 seconds. I got this procedure fro my MitiVac manual.