What is this brake thing?

[GoranL]

Hi all!

I´ve been working on my brakes on my -68 swb stepper, and need som help. It originally had a l6 and manual drum brakes, it´s been updated somewhere along the line and now has a 350+th350 and the brakes are disks up front and stock rear drums with hydroboost power assist.

Since I bought the truck last summer I had brake problems, because the rears were locking up before the fronts and I had no real power assist. So I have overhauled the calipers, main cylinder and replaced the wheel cylinders since one of them was leaking, and also one of the wheel seals were leaking, and that´s also been taken care of.

Anyway, today when I was lying under the truck I discovered this thing, connected to the rear brake line. It sits on the frame, on the passenger side just in front of the rear spring. What does it do? Do I need it? Does it hurt anything? It is not connected to anything, electrically or mechanicaly, I can´t understand what it´s supposed to do?