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 ?