It might be normal, might be a sign that your hose is corroding internally and clogging the valve. Squeeze the hose and if it feels soft and squishy, replace hose...and check inlet to the valve (if you have one) and see if it's clogged. If so, remove gunk...with dental pick, vacuum, whatever.
Also, just read that someone here rebuilt his pump...I assume that means replacing bearings on it. Something I might need to do someday, but doubt you do.