My car is w123 300D,

hydraulic brake is working properly when I depress it as normal or suddenly.Yesterday,I would like to check its system, so
I drove it with low speed and I depress it a little but several time before sharp depress.

The result was : It 's very hard (strength)to depress it and the car was not stop. It ran for a long distance.
Itseem to be the brake was not applied.

Is it abnormal ? How to solve this problem ?

It almost sounds like you used up your available vacuum by depressing the brake pedal several times before trying to actually stop. Check your vacuum source, the vacuum pump, for proper function. Using a vacuum gauge, it should be pulling something like 22" HG. Also, check the big vacuum line that runs from the vacuum pump to the brake booster, make sure it has no obvious holes/leaks. There may also be a check valve involved