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Michaellong
08-01-2018, 12:35 PM
I have a 2001 Audi A4 1.8t fwd. The brakes are hard in the morning, but normal if I let the engine idle for a minute. I have checked for leaks near the brake booster and found some broken hoses and bad check valves. These were all replaced. Now I am wondering if my brake booster is bad? The brakes do go down as soon as I start the engine. However, they are stiff and need a lot of force to stop the car. This only happens after a cold start.

walky_talky20
08-01-2018, 01:24 PM
During a cold start, available vacuum is at minimum (compared to any other idle time). This is due to the high idle rpm during a cold start. Higher target rpm means the throttle plate is open further, meaning less vacuum in the manifold.

The effect is worse if the car is an automatic because the brakes aren't just stopping the car, they are fighting "forward creep" of the Auto Trans. And at cold start (high idle), the "forward creep" is much more powerful than usual due to the higher engine RPM. This creates the perfect storm for an Audi that is very hard to get stopped.

I will say the issue you are describing is not uncommon on Tipronic A4's in general. I've experienced it many times on various Audi's- where you have to pretty much stand on the pedal to get it stopped. Once its warmed up even for a minute or two, the problem is gone. There are many factors at play here, but lack of vacuum supplied to the booster is the main one. This is one of the reasons that starting in 2001.5, all Tiptronic A4's were equipped with an electric vacuum pump to supplement in this exact situation.

Things you can do to deal with the problem:
- Pop the shifter into neutral when you are braking so you aren't fighting the engine to stop
- Let the engine warm up for a moment and come down from high idle before you select a gear

Things you can do to actually address the problem:
- make sure you fix all your vacuum leaks
- make sure your brake booster holds vacuum after engine shut down

Extreme things you can do if it really bothers you still:
- Add an electric brake vacuum pump (either aftermarket or swap in the pump, vacuum sensor and ECU from a 2001.5 AWM car to add the feature)
- Upgrade brake size or use a more aggressive pad material that works very well when cold
- 5MT swap that pig like it's your job.
- Sell it and buy a Camry (lol)

Michaellong
08-01-2018, 01:48 PM
I already have a Camry! I just wanted to know what I could do as my friend's 17 year old cousin is borrowing the car and his parents are worried that the brakes will fail one day and he will crash. Now I get why I never felt this issue on my manual 2001 A4. Is there a way to check if the brake booster holds vacuum after shutdown? If I remove the brake booster hose after ten hours, air rushes into it, so it seems like it is holding vacuum.

walky_talky20
08-01-2018, 02:14 PM
lol @ Camry

The brakes themselves are fine. It just isn't getting as much boost assist at that particular time during cold start.

There is one thing I didn't mention. There is a device that is meant to help with "creating" additional vacuum for the booster on a 1.8T. It is called the "Suction Jet Pump". It uses air flow of the PCV breather system and a venturi effect to create more vacuum than actually exists in the manifold. However, it is common for the oil and gunk of the PCV system to clog the Suction Jet Pump and render it useless.

I don't think it's effect is huge, but it must be measureable for them to include it on the car. Anyway, it would be worthwhile to check the condition and flow of the Suction Jet Pump (SJP). You want to make sure it is not clogged and is successfully applying vacuum to the PCV bleeder valve. And you want to make sure bleeder valve is flowing appropriately and isn't clogged itself. Pinching the line between the SJP and bleeder valve while the oil cap is loose should have some effect. That would be one way to possibly test it. Putting in a new SJP may be worthwhile in this case as well. Just to make sure its clean and working optimally.

Better Solution: Let said friend's cousin (?) borrow the Camry instead. Bam. Zero problems. ;)