Hello, sorry for me English but ??

After a motor problem we change the tot of the motor, all run OK.
But some time the motor stop if we push the break . The motor at this time
coming from 1000rpm to +/-850rpm for +/- 1second if you bush the break on
the same time the motor stop.
We don't find the raison, could you help me.
I don't find any manual for this car.
If you have a site to look for this information.

Thanks in advance
Roger