understanding all the lines in the fuel system

[dropanddrag71]

70 3/4 ton, automatic, 350
OK.. my mantinence goal is to locate and replace all the rubber lines in the fuel system. I replaced the short one at the fuel pump. I found one under the cab, about a foot long, replaced that. Now.... is there anything behind the cardboard that's between the seat and tank? If there is, must the seat come out?

The one under the cab, the piece sticking threw the floor, is that connected to the tank or part of the sending unit, or something else?

[GASoline71]

You need the short pieces of rubber line in the fuel line. It allows for flex in the vehicle from the cab to the frame, and then the frame to the fuel pump. You can run hard line all the way from the pump to the carb if you have an original Q-jet. If you don't use the short pieces of flexable hose... you'll develop cracks in the hard lines.

I used braided SS fuel line for the "flex" lines.

[ChevyCruizer]

I think he was still using rubber lines, just replacing them because of leaks or the possibility of leaks.

There is no rubber line under that cardboard gas tank cover. But yes, you have to remove the seat to get access to the pickup/sender.

There are small pieces of rubber line in the evap system, if you have that. It is in the passanger side of the rear cab. If you look back there you will see one bolt that is holding a fuel line that goes up into the corner near the back window. I took mine out when i relocated a gas tank. I did not really look at the metal line but it had a "U" upside down at the top of it. That might be rubber line, but I thought it was just steel with some rubber or plastic sleeve on it to keep it quiet. Either way this section of line mounts to the tank with small sections of rubber hose. I think if these were bad you would definately smell gas in the cab.

Also, every one of my trucks has had a bad fuel sock/prefilter. It is mounted at the bottom of the pickup. I may of had bad luck, plus most of my trucks (5 of them) have been in non operating status from anywhere from 5 - 15 years when I got them, so that might have something to do with it. But they sell replacement socks or you can just get an entire new unit. Hint - they come in both 5/16 and 3/8. Most 6's have the smaller while 8's have the larger.

[SS Tim]

Section 8 of the factory service manual has a number illustrations to help you. Best bet would be to start with a review of that section and see what your truck has and if it is stock or has been modified. Your question is worded such that it seems you are not at all familiar with the generaal system much less your variation. The seat back if not the whole seat really needs to come out. The hose extending through the floor is the feed line from the tank. It originates at the sending unit/fuel pickup. Don't forget the vent system too, but being a C20 you probably don't have the evap. recovry system to worry about.
If you haven't you can download the factory manuals here.
http://67-72chevytrucks.com/vboard/s...d.php?t=558016

Finally there is no real need to replace your lines beyond a quality rubber hose. But I by choice use a high pressure rated (fuel injection) rubber hose when possible it just seems to work a lot better.

[GASoline71]

Originally Posted by ChevyCruizer

I think he was still using rubber lines, just replacing them because of leaks or the possibility of leaks.

After re-reading his initial post... you're right.

Gary

[JointTech]

theres 2 pieces of rubber from the tank to the hardlines that comes out the bottom of the cab. one fuel and one vent. Then it goes to rubber where it hits the frame rail. fuel filter is there and just rubber on the vent side. hardline to right before the fuel pump. and some rubber from pump to carb.
at least my 72 was setup like that.

[dropanddrag71]

Originally Posted by ChevyCruizer

I think he was still using rubber lines, just replacing them because of leaks or the possibility of leaks.

There is no rubber line under that cardboard gas tank cover. But yes, you have to remove the seat to get access to the pickup/sender.

There are small pieces of rubber line in the evap system, if you have that. It is in the passanger side of the rear cab. If you look back there you will see one bolt that is holding a fuel line that goes up into the corner near the back window. I took mine out when i relocated a gas tank. I did not really look at the metal line but it had a "U" upside down at the top of it. That might be rubber line, but I thought it was just steel with some rubber or plastic sleeve on it to keep it quiet. Either way this section of line mounts to the tank with small sections of rubber hose. I think if these were bad you would definately smell gas in the cab.

Also, every one of my trucks has had a bad fuel sock/prefilter. It is mounted at the bottom of the pickup. I may of had bad luck, plus most of my trucks (5 of them) have been in non operating status from anywhere from 5 - 15 years when I got them, so that might have something to do with it. But they sell replacement socks or you can just get an entire new unit. Hint - they come in both 5/16 and 3/8. Most 6's have the smaller while 8's have the larger.

I have a clear filter at the Carb, fills up full when running, I just wanted to replace the small rubber sections as a preveative measure... BUT! The sending sock I didn't know about, safe to assume that since the filter fills up I should leave that alone?

[JointTech]

i hope its a plastic filter. Theres all kinds of stories on the interwebs about the glass ones breaking and burning cars to the ground.

[dropanddrag71]

Originally Posted by JointTech

i hope its a plastic filter. Theres all kinds of stories on the interwebs about the glass ones breaking and burning cars to the ground.

Plastic!
Note...... the lines under the truck were 3/8
Line from fuel pump to Carb is 5/16. Positive, just replaced the filter