Many pics included.

Here is the tank immediately after I got it out of the car. Pulling it was not too bad. Just a few things: The 3 rear screws for the tank cover have to come out. The tank support bolts do not have captured nuts above so you must hold them with a wrench. I also disconnected the muffler hangers and let them sag to get the tank out.
I read here that you let the tank down enough to get to the fuel lines and vac lines. Now I realize you reach down through the filler hole after you let the tank down in order to disconnect these and I did remove the filler door assembly for a little bit more room.



After wiping and blowing 45 years of stuff it was time to remove the 12 screws. The air ratchet was perfect for the entire job. Not sure why the only rust is on this plate. Maybe excess gas was spilled here through the years.


Easy Peasy! The sending unit lifted right out with a slight pry on one corner and looked just fine.


This is the only tank sticker in sight. LOL. Not what I was hoping for.



More pics soon to come.

 

The filter sock is trash. Change it even if it "looks" good.

 

Drop the tank cover. Your build sheet is glued to the top. It might be readable.
Why did you drop the tank? Bladder collasped?
If you pull the bladder, or replace the tank, you will need to keep the bladder neck that seals the sending unit to the tank.

 

These are photos of the inside of the tank. I was surprised at how clean it was and it was all drained except maybe a few ounces. (I had siphoned it with a cheapo Harbor Freight hand pump siphon.)
Before dropping the tank completely, I put this same vacuum pump on the bladder vacuum line and pumped all the air out of it, which apparently pulled the bladder back in place. I have no idea how badly it was collapsed but last time I drove it (a few years ago) it was not possible to put more than about 6 or 7 gallons of fuel in the tank.

I wiped out the tank thoroughly. The bladder material was still soft and supple with no apparent issues and no fuel caught between the tank and bladder. Below are the pics inside the tank. Straight down, to the right, and to the left.





The following shows where each of the ports are located. In trying to determine a root cause for the bladder collapse, it occurred to me that maybe the fuel vent was clogged. I attached a piece of hose to this port and drew air in and out with my mouth. It flowed in both directions, albeit with quite a bit of restriction because the tube is so small. I had previously used my vacuum pump from the charcoal canister line at the front of the car and discovered the line was clear when I could hear a bit of vacuum relieving at the tank. Maybe I cleared an obstruction at that point? Hmmm... not sure.


This is a pic of the cheapo pump I used to drain the tank and to vacuum air from between the bladder and tank. This is for illustration purposes only, as it will not draw vacuum after the sending unit is removed.

 

If you look at the “1976 Chevrolet GM Service and Overhaul Manual Supplement (ST-329-76)” section 8 (Fuel Tank and Exhaust System). it would seem that GM knew of at least one condition that would cause the bladder to collapse:

(QUOTE)
The vapor hose attached to the liquid separator also attaches to the equalizer pipe on the tank top which also incorporates a .035” orifice. The purpose of the equalizer hose is to keep pressure from building up between the tank and bladder and collapsing the bladder.
(UNQUOTE)

Then they talk about diagnosing the fuel tank or bladder:

(QUOTE)
If a condition of running out of fuel or a limited fill condition, 4 to 6 gallons maximum is encountered check for:

1. Collapsed, kinked, or disconnected vapor hoses at the canister or along the left
frame rail from the fuel tank.
2. ...
3. ...
a. Vent tube between bladder and tank for blockage.
(UNQUOTE)

That sure sounds like the problem most of us are experiencing!
The “Corvette 1975 Assembly Manual”, section 8A-1 (page 317) thru section 8B-1 (page 321), is a good source of diagrams. It shows where everything is located and how the pieces are supposed to go together.

 

Wow! Good info. I am trying to put this thread out there so others won't struggle with this issue in the future.

I am considering venting the tank and bladder connections separately. I see some others have done this. My thought was to vent the tank to atmosphere under the rear quarter and leave the vacuum source on the bladder port. What do you think of that?

Also, wondering what would happen if I just go ahead and get a vented gas cap. It seems this would help as well.

 

Yup. Found the tank sticker on top of the tank cover. All in, probably wasted an hour pulling the cover off and getting it back on. Just a big pain.

10/10 would not do again.

I guess I am glad to see it and curiosity was satisfied concerning the elusive 'tank sticker'.

 

My bladder had collasped to the point that it would not stay up against the walls. I was only getting a fill of about 5 gallons. So I doubt that any vacuum connection would be strong enough to keep the bladder to the walls. And what is going to happen when the vacuum goes away?

Why expend all this work on saving the bladder?. Take it out. If your tank is good cut the neck off the bladder and refit the sending unit.
Cars before 75 did not have bladders in the tank. They seem to keep on going despite all the nanny warrnings we are going to burn to death without the bladder. The bumper suport is at least as strong as the earlier cars and we got the shield.

I don't like the smell of gas in the garage. So I would keep the non-vented cap and let the charcoal vapor cannister do it's job.

 

Yeah, but if you didn't go for it, you would keep thinking about it......

 

I hear ya', but I like the thought of the bladder. If it fails again, i will not hesitate to dump it. At least I am familiar with dropping the tank now. Less intimidating since I have done it once. I bet now I could do it in an hour if needed.


Haha, for sure. Now I know. If I had a valuable car I might do it again, but for a base stocker like mine, nope.

 

I think most of us have more time & money invested in our cars than we can ever hope to get back.
My build sheet was a better read than yours, except for hole in the center where it had been glued to the cover. But yeah, having it won't add any money at the end.

I had posted a walkthru whitepaper on this topic back around 2004 or so, but I can't find it using the search function.

One thought on the vacuum thing to keep the bladder from collapshing. Are you going to fill the tank with the engine running so you have vacuum? If not, then that process seems doomed to fail.

 

It worked from the factory (mostly...) with vacuum and did not require running the engine during refueling. Do you mean if i modify it?
I was just out looking at my evap canister and plan to ensure it is getting vacuum to the tank as soon as I get this thing running. Then a final decision can be made.

For now, maybe I'll pick up some fuel hose and relocate that ported tee to an accessible spot.

 

Yes it worked from the factory to now - 45 years. In the meantime the bladder material has been sitting in gas, and of course the new gas is not good for any kind of rubber material.

GM only used this rubber bladder material for a couple of years, 3 I think, until they went to a stiff liner. Hmmmm, wonder why?

But since it has been weakened and was in a collasped condition for what you said has been a couple of years, I would expect it to collaspe again.
The vacuum thing was not designed to hold the bladder upright, nor will it ever be strong enough to keep it upgright.

Good luck and I hope it works out and you don't have to drop the tank again.

 

So in continuing to find a root cause of the fuel tank bladder collapse problem, I found something unique but possibly very important.

Below is a pic of the vacuum line that goes to the charcoal canister, It was only pushed onto the carb vacuum port with no clamp and damage to the hose. Certainly not how it came from the factory. The pic shows the damaged hose and the port just to the right. (The hose is being held up about 6" from the port.)

Someone has definitely been here before and I suspect they may have been attempting to repair the same issue I am experiencing.

I also attempted to create vacuum from this hose through the canister and could not detect vacuum at the port that leads to the fuel tank. I may be pulling that charcoal canister to have a look.

 

Your bladder looks to be in excellent condition!
Your canister should have a removable service cap with a open drain nipple at the bottom so pulling a vacuum like your trying to do is futile.
Just keep the fuel tank near full, especially when the vehicle sits for an extended period of time.
IMO, this is why 75-77 rubber bladders do collapse.
There is a section In the 77 service manual talking about bladder tank dry storage. That it is necessary a thin coating of 10wt non-detergent oil be applied to the inner surface of the bladder as a preservative.
Yea, like that is going to happen each fall....

 

I thought it looked and felt pretty good and I was surprised how clean it was inside.

So exactly how does that canister work? It looks simple, vacuum in from the carb and out toward the tank. I have looked around to attempt an understanding, but the modern systems have a purge cycle that I seem to not have. I am considering just bypassing that thing or leaving the vacuum only on the bladder nipple.

 

Or every 45 years.