Need help settling a debate

[meatbag]

So I'm driving my plow truck, notice my brakes are weak as f, then driving up hill it starts backfiring like crazy, almost didn't make it. Unfortunately I'm working with my father on this project. So I figured the brake booster went to crap and it caused a massive vaccum leak, like diaphragm ripped or something. I switched brake booster and same symptoms, I figured it was a vaccum leak. hooked up vaccum gauge, looked like it had plenty of vaccum,. I still sprayed wd 40 to check and pulled carb to put new gaskets. Couldn't find a leak. So I said it has to be a vaccum leak, and that it's probably on the rear of the intake manifold. He's saying it can't be, because it has plenty of vaccum. So is it a vaccum leak or something else.

 

[squaredeal91]

Im not sure of brake issue, might be unrelated. Look into your distributor for the running issue. Possibly the vacuum advance is bad or distributor advance weights are stuck.

 

[Jawzjeep]

What was 'plenty of vacuum' in a number form? Where did you connect the vac gauge or what port? and did you disconnect and block the vapor canister?

I'd make sure your testing it properly and get us a number.

 

[Turbo4whl]

What @squaredeal91 said. Then check the internals.

Need to check the basics, cylinder compression, leak down test. Your symptoms could be a wiped cam. Pull a valve cover, are all the valves moving the same amount?

How many miles on the engine?

 

[SquareRoot]

Auto trans? If so, check the modulator. Mine was t-d into the vacuum system and went bad and caused similar issue.

 

[meatbag]

What was 'plenty of vacuum' in a number form? Where did you connect the vac gauge or what port? and did you disconnect and block the vapor canister?

I'd make sure your testing it properly and get us a number.

It's between 18 and 20

 

[meatbag]

Auto trans? If so, check the modulator. Mine was t-d into the vacuum system and went bad and caused similar issue.

Interesting I'll look into that. I was thinking about throwing a inline check valve to the brake booster line, might help

 

[squaredeal91]

@meatbag Any progress made?

 

[Rickf]

If it's an HEI, could it be the ignition module (not related to any brake issue) ?

 

[Matt69olds]

Check the wires on the pickup coil. The constant back and forth movement of the vacuum advance will bend the tiny wires and cause intermittent no spark. The engine will continue to run, just load up cylinder’s with fuel. When spark is returned the fuel ignites with a bang.