2nd Gen Non-Engine/Transmission Proportioning Valve?

[tjm]

I have a 96 4x4. On the rear axle where the brake line tees off to left and right. There is a tube that is about a foot and a half long that comes out of the tee and goes straight up to point toward the bed. on the end of this tube there appears to be some sort of cap? Does anybody know what this is and what it does?



Thanks in Advance.

 

[jabird]

vent tude for axle

 

[tjm]

thx jabird