As you alluded to a blow off valve releases extra air when the throttle closes suddenly. More accurately, it releases extra pressure to prevent damage to the throttle or other components. An aftermarket blow off valve can make more noise or be necessary if other modifications allow for higher boost.

What you need to look for is engine codes or other problems. The blow off valve usually recirculates air into the turbo to prevent inaccurate readings on the mass air sensor. By venting to atmosphere the blow off valve makes loud noises but also releases air that the engine doesn’t know is missing, which can cause issues.

Try driving and suddenly letting off the throttle while under boost then quickly getting back on throttle. If the engine stutters or stumbles then the engine isn’t tuned for the aftermarket blow off valve, if it doesn’t have an issue getting back to power then it should be okay.

It sounds like a brake issue. The tire is independent from the others unless it’s awd so the tire size, tread, and speed rating won’t make a difference, especially if the tire isn’t hot.

There are a lot of issues that could be causing this. The most common is if the parking brake is too tight or the slide pins are sticking or seized. You can get an infrared thermometer pretty cheap. Just measure the brake rotor temp after driving a while. If one is more than the others by more than30-40 degrees there is probably a problem. Keep in mind the front should be higher than the rear.

There may be load on the parking pawl. If you can, try jacking up one of the front wheels while the parking brake is set. That should get the load off the pawl.

You can also see if it has a little plastic cap near the shifter. Remove it with a pick or pocket screw driver and check to make sure the solenoid is actually fully disengaged. Sometimes they stick just enough to keep it locked but still make a sound.

Try arp. They probably have something.

First, you should never refill brake fluid without first addressing the cause of the leak (unless you’re diagnosing the leak obviously). If the abs module is the leak then yes, that needs to be replaced.

Second, you would have had to lose a lot of fluid to break an ABS module. They are generally just there until they are needed. Water in the brake fluid can corrode them but if you don’t have a dash light I would get a second opinion before you replace it, it sounds like they’re trying to rip you off.

Third, as to your original question, ABS is only important in bad road conditions or emergencies 99% of the time you won’t notice it not working. If it’s not leaking then just remember that you won’t have as much control braking in those conditions. I wouldn’t call it safe but you won’t lose your brakes out of nowhere if it isn’t leaking.