The differential housing has a breather vent cap on the top in order to prevent pressure from building up inside. It's not a one way valve, it's just a hollow bolt with a cap to prevent moisture or debris from getting in. Importantly, there's a baffle on the inside of the housing that goes around the bottom of the breather. If you're swapping your differential housing to a stronger RX7 unit or another Miata housing, make sure that this baffle around the vent cap is in place.

Vent cap:



Housing with baffle in place:


Otherwise, you'll get a surprising amount of diff oil coming out of the breather valve.


Here's a housing without a baffle or vent cap. If you're purchasing an RX7 housing from a junkyard you might find that it's missing both.


The baffle:

It's held on by one bolt.

This might be obvious to some, but I missed it and wanted to post to potentially save someone doing a differential housing swap from making the same mistake.