Many of the Saturday night racers use automatic transmissions and the simple fact is that these are, and should be, a sealed unit. If they are leaking you need to locate the source of the leak and fix it. If you are running a manual transmission you can generate some serious pressures in the gearbox and should consider a vent. If the gearbox has a pump that routes gear oil to a cooler, you still should consider a vent that utilizes a filter to keep dirt or other debris from entering the transmission.
The reason that we need vents in the transmission is that the rpm levels the engines are running are substantially higher than the transmissions were designed to see in the original applications. Even gearboxes that were designed for racing should be vented to prevent built-up pressure in the box. The gears spinning in the oil produce a good bit of windage and that can cause pressures to build much the same way that the engine can produce pressures in the crankcase, and this can cause oil leakage at gaskets and seal interfaces. Not a good thing. The same logic that applies to the engine breathers also applies to gearboxes. The vent should be routed through a filter and the vent needs to be located above the oil level.
The concern here is not so much contamination but physical intrusions into the brakes and specifically between the brake, wheel, and disc interfaces. Such intrusions can be avoided by building a small shield that will prevent anything from entering or getting trapped between the brake and the wheel or from entering the caliper. There are also some shields on the market that fit between the wheel and the brake that help prevent anything hard from going through the wheel into the brake. These shields also help to minimize brake dust from getting on the wheel.
If you run in a class where pit stops can be an issue, you want to avoid the possibility that a lug nut could get caught between the wheel and any part of the caliper or disc. If a lug wedges itself between the wheel and the hub after a pit stop, the lug could cause a wheel to loosen or damage the wheel and the brake, ending your day at the track on a sour note. Brackets to prevent this are easy to make and some are available from brake vendors.
Keeping dirt out of your fuel system is not as easy as you would think. The best way to keep the fuel system clean is to not allow anything but fuel into the system in the first place. (Thank you, Captain Obvious.) While this is easy to say, it is a bit more difficult to accomplish. The first place to start is with fuel transfer equipment. Keep your fuel cans clean inside and out. Make sure that the area around the fuel inlet is clean prior to even opening the fuel cap. This is an easy thing to let slip if you are racing a dirt car. Mud is usually everywhere, so take the time to clean the area around the fuel filler prior to even opening the cap.
It doesn't hurt to run a filter in the funnel so any dirt can be caught prior to entering the fuel system. If you are running a stock fuel tank on your Street Stock or Bomber class racer, take the time to remove the tank and clean it thoroughly. If not, the loads and the shocks that you will be placing on the tank will break loose any rust and scale that will have formed in the previous 15 to 20 years the car was used as a passenger car. The amount of rust scale and general debris that can accumulate in a fuel tank over time is significant.