Wadw this will explain what the part on your firewall is:
We all have seen the dual action fuel pumps but here are two other Chevrolet/GMC applications to make those wipers work better. First is my Windshield Washer Coordinator Group No. 8.798 PN 986984 this one is from 1954 and is for passenger cars I think it would work on a truck except for clearance issues. It allows the windshield washer bottle and wipers to run together. You "T" off the bottle and the vacuum cylinder attaches to the motor linkage on top.
http://good-times.webshots.com/photo/2475288740102454676qzHoyeThe next is a Trico set-up that is mentioned for trucks but only GMC specifically (Booster W/S wiper vacuum 23.3787 PN 2233295 1939-53 Parts Book Light Duty) although the Chevrolet Truck Parts Catalogue 1942-1956 mentions Trico as the service provider. I have owned two trucks with this set-up attached to the firewall both were Chevrolet and both were 2-tons.
http://good-times.webshots.com/photo/2629558360102454676CIHAvyPicture here shows where the unique VAC switch was mounted under the dash.
http://good-times.webshots.com/photo/2145875240102454676VbBPfpHere is the complete Trico (Electro-Vac) kit.
http://good-times.webshots.com/photo/2663868800102454676yRTvVe I don't own this but the owner was kind enough to sell me the large instruction sheet which I will try and scan for posting later. Anybody running one of these on their bolt? There are several detailed pictures of both items if you follow the links.
