Booster going bad or something else?

[JoshT]

As part of converting my 68 F-100 4x4 to front disc I opted to used a dual diaphragm booster and master cylinder off of a later mid 70s F-250 or F-350, parts were actually for a 74 application IIRC. Things had been working pretty good until some other issues cropped up recently that don't seem to be directly related to brakes.

Few weeks ago truck started bogging bad under moderate to heavy acceleration. Turned out to be a combo between a clogged fuel filter and particulate on the screen for the fuel pressure regulator in the Holley Sniper EFI. They were affecting fuel pressure in separate ways and causing separate issues. That's fixed now and truck is back running good.

Same time as that was being diagnosed and fixed I started noticing something weird with stopping the truck. When applying the brakes to slow down or come to a slow stop everything feels normal. If I get on the brake pedal harder the pedal gradually gets harder to the point it feels almost like a bad brake booster. Also when it gets like this, or if I'm pressing the pedal hard sitting at a stop, the engine will start popping through the exhaust. Kinda like backfiring, but not as harsh as I usually think of a backfire being.

It only happens when on the brakes like that, as soon as I ease up on the pedal it stops. Due to that the only thing that comes to my mind is a booster issue, but I am not familiar enough with dual diaphragm boosters to be sure. Only time I've delt with a bad booster was a single diaphragm and it was straight up bad. Is my booster failing? Something else going on? Any way to really test a brake booster?

[390FE1972]

Here is a quote from someone here on the website that was talking about the brake booster and engine performance: "If the diaphram inside the booster ruptures. instant vacuum leak to your intake manifold. thus causing engine performance issues like running lean, induction backfire's. etc etc. if yours is hissing then its probable you have a defective booster."

Hope this helps!