Vapor lock or a bad fuel pump? Any thoughts?

I have an AC mechanical fuel pump with a glass filter globe. Yesterday it was around 85*. My wife and I took the truck to run errands. It ran Great to the vets office. I stayed outside and checked over the truck while she went in. I left it run while I looked around hoping the airflow would help cool it off. I noticed a steady stream of large bubbles (like boiling water) up through the fuel from the inlet port in the base of the fuel pump. Shortly there after we left and the truck wouldn't accelerate, nor would it maintain under load at more than part throttle in 4th gear. I managed to baby it home at lower speeds/gears. Today I dropped the tank and pulled the fuel lines. I found crud blocking the fitting for the soft line. I blew out the lines and drained the tank. I reassemble the lines including the dip tube and vac tested them as an assembly. It held 15" of vac just fine. I reassembled all of it only to still see large bubbles- less frequently, but they are still there. The doesn't seem to keep the fuel level over the built in screen either. The level drops the longer the truck is left to idle. Raising the RPMs raises the fuel level only slightly though. I haven't tried to test the pump yet. When I removed the upper section earlier the diaphram appeared to be intact- it's brand new from the recommended fuel pump expert on the east coast- can't recall the name... Now and then parts or something?

So what do you think- bad

If it were mine it would get a high quality low pressure fuel pump mounted close to the tank and wired through an oil pressure switch.
Fuel systems are one of those things that can be plugged and leak at the same time. Oh boy.
All it takes is a tiny pinhole aft of the mechanical pump to mess everything up. Try drinking your soda with a hole in the middle of the straw. You get more air than liquid. Same for mechanical pump systems.
Mix in ANTIQUE STUFF and you can see what I mean. Trouble just waiting to happen.
I suspect the air bubbles in your pump are the result of a pinhole. On the other hand, I guess it could be boiling fuel, too.
Just to try something, buy a high quality low pressure fuel pump, mount it close to the tank, wire it through your ignition switch on or a toggle switch for now. Did your problems go away?
Pressurized fuel is less likely to vapor lock, too.
I know I sound like a smartass. I apologize. Some folks here cast a wary eye at anything non-stock. Funny, huh?

Good stuff, SG, from someone who knows. Don't apologize. Right is right.
Getting the job done with reliability becomes more important than looking all original if the option is failure. IH worked with the affordable technology, back in the day. In the 1950s there was a service station every few miles because it was necessary. Our family had one of them stations.
One option that I like is an electric pump and a residual check valve in the fuel supply. The pump can move more fuel than the engine can use and the over supply is returned to the tank when an over-pressure situation happens. The returning fuel is cooled by mixing in the tank. Like SG said, pressurised fuel is much less likely to boil ( bubble) when the operating temperature in the engine compartment gets really hot. I like to have a simple check valve in the fuel supply line. this will prevent leak-back and better, quicker starts.
Another option, Join an auto club, like AAA. Make sure you have towing home in your agreement.

Today I dropped the tank and pulled the fuel lines. I found crud blocking the fitting for the soft line.

It's possible that the crud was causing your vapor lock or at least multiplying the problem. Remember that when it comes to liquids, increasing the pressure of a liquid increases the boiling point. When you start putting a vacuum on liquids, it LOWERS the boiling point. So if you fuel pump was having a hard time pulling fuel to it, it may have been lowering the boiling point of your gasoline and caused vapor lock. BTW, do you have access to gasoline that has 0% alcohol? That will also help. (We've got it where I live, and it used to be only a nickle higher than E-10. But as soon as the weather warmed up, it's now about $.15/gallon higher)

If your gasoline is boiling when it's pretty much straight out of the tank...which is what it sounds like you're saying...then maybe you need to check how your exhaust is routed. [That was a joke.]

Thing is, if your gasoline is trying to boil at an ambient temp of 85 degrees, after only passing into the fuel pump, my first thought is to ask where it's picking up the extra heat to make it boil at that low a temperature. Had the fuel line run right next to the engine, rather than up the frame rail, that MIGHT make a little more sense to me.

Re-check your fuel lines. Pappy had a '70 Plymouth Volare with a 318 that kept killing fuel pumps. After the 3rd one, the guy at the parts store said he wasn't going to replace any more of 'em...so we started looking for an external cause. I'll be darned if I didn't find a bad spot in the fuel line, where it traversed the front crossmember. There was enough grease, dirt, gunk and junk there to keep the line from leaking...but every so often, a piece of the rusted fuel line would flake off and puncture the pump diaphragm on its way to the filter. SO...if you're having a similar problem...those bubbles at the pump just might be air entering the line, instead of boiling fuel.

Just as Smog Guy suggested.

All of this is good info and is confirmation of where I was headed. After pondering this some more- The bowl is under vacuum so the diagram is on the discharge side of the pump. That being said it has to be in good shape. That leaves the upper housing possibly being cracked/pourous, a bad brass elbow, or a bad brass JIC to barb fittng as I've tested the rest outside of the truck. All of the steel fuel lines are original and in their original locations. I replaced the flexible line between the tank line and the pump. The muffler is on the pass. side of the driveshaft within the frame rails. The tailpipe is directed out under the pass corner of the cab.

The crud I found blocking the soft line fittings was soft material; dirt basically. I recognized it from when I dropped the tank and sent it out for boiling/repair. I thought I had found the source of my concerns then. Hence back flushing the complete assembly and draining the tank.

I do have a small solid state 12v pump laying around. I'll try fitting that today and see if that helps the situation. If that's the solution I'll spring for a nice carter unit or similar. Right after I do a vac test on the inlet side of the pump.

It's interesting that ethanol fuel was mentioned. I originally bought 5 gal of "recreational 90" fuel (non-ethanol) as I wanted to keep the fuel system free of the ill effects of ethanol. In my haste of wanting to drive it around instead of taking the other car on a 45min trip to get more non-ethanol gas I filled up at the local station and dosed it with stabil ethanol treatment to "storage" levels 1oz/5gal. I felt that 1 tank wouldn't do much harm. Maybe I was wrong...

I have never heard of IH trucks having problems with vapor lock. I run all mechanical pumps on mine and they seem to work fine. I did finally give up on the original fuel tank in my R160. After numerous cleanings of the original tank, and filter changes, I bought a plastic 6 gallon tank that sits on the floor of the passenger side. Not the ideal solution but it ended my fuel problems on that truck. My other trucks must have better tanks as they still work fine with the original steel tanks.
Once my S160 stalled for lack of fuel and I eventually found a small piece of rubber lodged in the fitting at the inlet side of the fuel pump.

If I missed that you had replaced the metal lines, I'm sorry. Replace those old, rotten, bent, twisted, plugging, rusty, porous, dangerous metal fuel lines.
No fine wine thing going on here. They're toast.

Adding an inline filter between pump and fuel tank will catch any tank sediment. As SG mentioned Jake, best to just replace the original fuel line. You may want to reuse the fuel line fittings as the original ones are different than those on current brake and fuel lines. Will match up to your current set-up. At least that is what I found to be true. You may have a hidden pin hole in the metal brake line that is against the frame.

I vote you're sucking air... I saw similar condition on fuel bowl on my marine engine. The pinhole in fuel line did not leak fuel out but could suck air in. I have noisy carter electric fuel pump on my v6 because the engine does not accept a manual fuel pump. I would much rather have a manual pump, more reliable IMO and no noise. Could be any connection in fuel line whether at pump or tank.. Good luck, you'll find it the last place you look