If a hot differential is cooled rapidly, by going through water for example, a low pressure area will be created inside the differential. Oil seals are designed to seal in one direction only, so if the low pressure inside the differential cannot be equalized quickly, water will be drawn past the oil seals and into the differential, which can have serious ramifications.

Many four wheel drive vehicles are fitted with differential breathers by the manufacturer, but these might not be ideal for serious four wheel driving. The biggest problem with OEM breathers is that they may not be positioned very high off the ground. If you plan on doing river crossings, fitting aftermarket breathers higher up on the vehicle is highly recommended.
As well as the differentials, the gearbox and transfer case will probably also have breathers. Including these in your breather repositioning design is something that should also be considered.

When the coil suspension was done on my truck the differential breather lines ended up being about 700mm off the ground. That’s a reasonable height, but I wanted them to be a bit higher than that.
Initially I looked at the commercial breather kits that were available; eBay has quite a few offerings. To do the two differentials, the gearbox and transfer case, a suitable kit was going to run at about $140.00, which seemed quite expensive for what was actually included in the kit. The other issue I had was that none of the kits I found online really suited how I wanted to do the installation. So… I chose to make my own.
I also decided that this breather system should include the atmospheric balancing lines of the tank pressure senders, but I’ll go into more detail about that in a separate article.

My custom breather design included the following items:

• 8 x 8mm push lock fittings
• 6 metres of 8mm nylon air tubing
• 1.5 metres of 6mm rubber fuel hose
• 4 x Stainless steel hosetails (2 x 6mm and 2 x 4.5mm)
• 1 x sintered bronze air filter
• 4 x custom made aluminum manifolds (to connect things together)
• A stainless steel bracket and weather protection plate
• Associated stainless steel nuts and bolts

The breather manifolds, like the ones I made for the truck’s compressed air system, are machined from 25mm square aluminum bar. All of the threaded push lock fittings are 1/4″ BSP and the sintered bronze air filter is 3/8″ BSP. I prefer not to use nylon tubing where the tubing is constantly flexing at the push lock fitting, as this can affect the longevity of the O ring.
Instead, I choose to use rubber fuel hose to connect the the breather outlet on the differentials up to my breather manifolds. Nylon tubing is then run from the differential breather manifolds to the main breather manifold.

The gearbox and transfer case breather fittings were installed previously, when I undertook to re-engineer some of Fuso’s engineering.
At that time I simply connected the existing OEM breather pipework to the push lock fittings (using a short joiner), but this OEM braided rubber hose has now been completely replaced with the 8mm nylon air tubing. There is sufficient nylon tubing between the gearbox and the chassis to allow for movement of the engine. Cable ties and P clamps are used on the gearbox to secure the nylon tubing, preventing any significant flexing at the push lock fittings.

The main breather manifold is located near the passenger’s side cab latch mechanism and is positioned under the rear lip of the cab, giving it protection from the weather. It is attached using a stainless steel bracket I fabricated for the purpose and a protective cover plate was also used to give more weather protection to the air filter, not that this is really needed. This location positions the sintered bronze air filter 1500mm off the ground, which, unless I repurpose the truck as a boat, should be plenty high enough.

The dollar value of this installation probably worked out to be about the same as one of those eBay breather kits, but that obviously does not include the manufacturing costs of the bits I made. That said… my time does not really cost me any anything these days, so fabricating the manifolds and brackets did not really factor into my decision making processes.
At the end of the day, I ended up with a completely custom breather installation that I am happy with. What more can you ask for?