Help: hard brake occasionally

my 1995 lumina apv has a problem of brake pedal effort since this Monday for 3 times to now.
The brake pedal becomes very stiff suddenly. The first time, I kick it, it was smooth as I hear a click sound. The second and third, it was ok after I step on the gas. It just happens occasionally. I really don't understand.
As someone knows it the brake booster, i.e. worn diaphragm, runs out which will cause this matter occasionally? Or vaccum lining, brake lining, check value nalfunction....
My old car has problem of its rear drum brake. I think both of them were worn. I know it, so I use the front disc brake to stop the car. Recently, one of the drum was fratured. this thing can cause hard brake pedal occasionally?
urgent helps!!! what is the problem? why it happens occasionally?
many thanks for any suggestion.

By the way, all these were happned after I changed the winter tires. Is there something wrong?

I would check for vacuum line that could have a crack or jumped from the brake booster or intake manifold. Effort does not relate to worn brake pads or shoes nor winter tires. Do you have idle problems or irregularities? What engine type do you have?
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YOU said: I have it checked.
...WHat is "it" ? .....vacuum line ?

you said: ... there is a idle problem, maybe. When I stop at crosssection. The engine is out of breath, and the engine light is on, the brake becomes a little stiff, and then stalls. I can see the needle of battery swings. Everything is ok when I restart, the engine light is gone out too.
..THIs would still be relevant with an opened vacuum line ....it depletes the brake booster from vacuum (hard pedal) , provides extra unmetered air to the engine, hence lean mixture, coughing and stopping, and slow rev of the engine runs the alternator slow enough so that it delivers less than 12 v, hence fluctuating voltage needle....all is relevant. So please confirm all vacuum lines are checked connected and tight ...
Another thing, there was a strong gas smell at air filter. I always think there was a leak, but I don't know how to check it.
..the 3.1 is a throttle body injector system...there are 2 injectors right there at intake mouth... there have been cases of injectors that blocked opened, and that can flood the engine at idle, and make it stop, and a backfire or simply excess fuel evaporation in intake manifold can feedback fuel scent into the air filter duct...
This would be accompanied by dark exhaust and unburnt fuel smell at exhaust...and abnormally low gas mileage...

....a damaged throttle position sensor could mimic most of those problems too....

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