I think you have a hydravac. I believe it was an option on 1 ton trucks. My '57 has one. You can see it in the lower right corner of this photo. http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/7f5847c2.jpg
If you search hydravac you should find some info.

Last edited by jfnar; 08/23/2010 8:51 PM.

'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc