What is this?

[EastMT]

Is this a one way valve? 
Is it necessary to use one?

 

[Jammer75]

Residual valve. Shouldn’t need it if everything works correctly. Helps from losing brake pedal psi. 
 

https://www.amazon.com/Wilwood-Engineering-260-13783-Residual-Fittings/dp/B01GVIM8G8/ref=asc_df_B01GVIM8G8/?tag=hyprod-20&linkCode=df0&hvadid=309898128610&hvpos=&hvnetw=g&hvrand=202770285087244999&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9030031&hvtargid=pla-566643848587&psc=1

 

[EastMT]

Thanks, I will keep it in there.

 

[L.R.S.]

Residual valves give a more positive brake pedal feel because they keep 2psi or 4psi on the system. I've never needed them,  but lots of cars I prep have them installed.

 

[EastMT]

Residual valves give a more positive brake pedal feel because they keep 2psi or 4psi on the system. I've never needed them,  but lots of cars I prep have them installed.

I'll run it for the first trip and see how it goes. Thanks for the feedback.

 

[honEXduner]

I'll run it for the first trip and see how it goes. Thanks for the feedback.

Also helps if your pads travel back too much. My old car with only rear discs would take an extra pedal stroke to pump them back up before getting firm if I hadn't applied them in a while, this valve would have likely prevented that.