If your vitz 99-04 is on cng and you are facing this problem but on petrol it seems to be working fine then the only problem with your car is its tuning on CNG.
This happens when there is less quantity of cng going into your car due to which your engine doesn't generate enough power and hence the car's brake vacuum doesn't properly work when there is load on the engine i.e. your AC is running in bumper to bumper traffic.
You might have noticed when you give it some throttle while your other foot is on the brake paddle, the brake paddle subsequently starts to get soft and you are able to press it.
I've been driving a 2004 CNG-ed vitz since more than 2 years now. I was facing this problem for sometime but then I learned to tune my car on cng , Vitz and other auto cars require frequent tune-ups on cng IMO.
Both the following problems in vitz are related to CNG.
1. Not being able to accelerate the car from a stop position while the AC is running.
2. Not being able to break when AC is running or there is load on the engine.
You first need to make sure your air filter is clean, plugs condition is good and the gap is right.
Note: This is going to be a bit tricky so do it if you feel you can do it otherwise go to a cng wala and tell him to do it for you as per your instructions.
Below is an image of a Landi Renzo kit. The "T" bolt is responsible for you car's acceleration and hence economy as well.
Picture source: Pakwheels.com
1) [Engine running. AC switched on. Gear: Parking/neutral] <engine running="" and="" ac="" switched="" on.="" gear:="" parking="" neutral=""> Pull the accelerator cable with your hand one hand and hold it there at one position (constant rpm, can be at any point but highest possible if not the peak point)
2) With your other hand while the engine is running at a constant high rpm. Rotate the Peak Speed Adjuster clockwise and you'll notice as you move on the engine will loose rpm. Now keep your ears wide open and listen to sound of engine carefully. Rotate it back anti-clockwise and you'll notice the increase in rpm..keep rotating it until at a point where you notice no increase in the rpm and rotating the Peak Speed Adjuster clockwise doesn't make any difference. THIS is the optimum amount of cng required for your car.
(Any further increase from this point will increase the amount of gas flowing into your engine hence more gas being burned. You'll have some what better acceleration but poorer economy)
3) Leave the accelerator cable.
Now again the same process with the idle speed adjuster but your don't have to pull the accelerator cable for this. Its recommended that you do this while your AC is switched on.
1) Rotate it clockwise and your engine will start shaking more and turn off if you keep rotating it but don't do that :-#
2) Rotate it anti-clockwise and as you move on, the engine will start running smoothly. Keep rotating it to a point where engine stops shaking and feels that it is running freely. (you might want to to check inside the cabin if the steering is vibrating. If yes then rotate it a bit more anti-clockwise) That is it
Go for a test drive. You should not be facing any problem now but if you still do then loosen the Peak Speed Adjuster and Idle speed adjuster a bit more.
Please do not curse me if you still face this problem. There might be something wrong with your car or the cng kit.</engine>