I finally got my bike repaired by allbike.
They found out the rear brake has no problem, but the front brake cause the problem.
The front axie has been replaced, front brake has been rebuilted and air filter has been changed too.
But now I found out there is another problem.
When I come to a full stop, I can not shift the gear up to nentral.
It stays in 1st gear.
I would like to know does this problem common to older bikes.
Does anyone knows how much does it cost to fix this problem.