Been working on my 72/78 f100 4x4 with the 300 6cyl. Pulled engine trans and t case, re installed everything after doing some work and now barely spins over with a good battery and within 10 seconds 1 of the brake lines coming from the master heats up and starts smoking. ( hot to touch ) anyone had this issue or something similar to it. It's grounded to the block and body.

 

Loose or missing ground cable for sure

 

Check/chase your brake lines from the M/C for touching/pinching a "hot" wire....and check the insulation on those wires.

 

I have replaced brakes calipers wheel cylinders and almost half the brake lines. Every thing I have looked at I haven't seen any bare wires touching on the frame or any of the lines

 

Hook up a ground from your engine to your cab. I've never heard of the brake line heating up before! But I have heard of automatic shifter cables frying and seizing in their insulations, due to becoming the body ground by default.
Your battery negative cable runs to the engine block? If it runs to the inner fender, then you need a heavy cable from the inner fender to the engine.
Running the negative battery cable to the engine allows you to run a smaller cable to the body, because the starter is about the only current-draw which requires a fat battery cable-sized ground.
Usually the body ground on Fords is an uninsulated braided flat cable running from a bellhousing bolt to the firewall, where it's attached with a small pointed bolt.

My truck is a '77, but when I got it, the cab ground was missing, and the only symptom I noticed was a bad starter run-on condition. Once I noticed the missing ground and installed one, the problem went away. (Though I changed the solenoid at the same time anyway.)

 

The symptoms described tell the tale.

The starter is the highest current draw by far, somewhere around 200 amps DC @12 volts, it is obviously "hunting" for a ground. It doesn't care where it gets it, but it might get the current it needs through something not designed to carry electricity. This is one reason why good fresh cables of adequate gauge, tightened securely, are so important. Shift cables and throttle cables are other usual culprits.

Make sure there isn't any paint interfering with any of the ground points or cable connections too. Three main cables - from (NEG) battery terminal direct to engine block, or maybe a starter bolt. From block to frame. From block to firewall.

 

A conducting brake line, that's different.

My suspicion is that something hot is grounding through the Dual Brake Switch.

Follow through the ignition sequence:


Yeah, I wrote this up for a 1970, but your's should be close enough.. maybe some of the wire colors are different.

It almost sounds like it's shorting at the ignition switch.

Assuming that it wasn't doing the same before your work, what work did you do that might have impacted any of the wiring?

 

Ground battery to block.
If the ground is put to the frame or elsewhere on the chassis, starter current has to find paths as descried above. Like through brake lines.

 

don't forget the Ground strap (from block to frame ) Left off I have seen strange electrical issues

 

2X. Funny how 12V DC electrical gremlins are usually ground related.

 

I know you guys love to get together and rattle the coffee pot but there should just be a sticky questions like this get sent to.