Is my brake booster going out?

[FormerMarine1981]

I did a search and didn't come up with any results. I was trying to crank my 72 today by poring gas into the carb. The engine would die after the fuel was used but every time it would crank the brake pedal would move on it's own. Can anyone explain what causes that and let me know if I need to look into replacing anything? Thanks, Ben

[BRUTUS_T_HOG]

if you had your foot on the brake don't worry about it.

if it truly moved on its own then yes the booster is leaking internally and should be replaced

[FormerMarine1981]

That's what I was afraid of. It was moving on it's own.

[DCrusher]

Has anything been changed or adjusted on the master cylinder or the pedal rod? Because unless the pedal is push which closes a valve which then starts to draw a vacuum on the booster if the diaphragm was tore or ripped then it wouldn't go down it sound like the pedal is holding the valve shut which causes the vacuum draw on the booster make sure the pedal is not sticking and in the full up position

[FormerMarine1981]

No I have not adjusted anything on it. The brakes had been working fine. Once I get it running I will update on how they are acting.

[fordman]

for now plug the vacuum line to the booster until you get the truck started and running. that will eliminate a possible vacuum leak fo ryou. . once you get it running hook the vacuum back up and step on the brakes and see if yo uhear a hiss when you step on the brake pedal. if you hear a hiss the booster is leaking.

[R.Smith]

Also look at the check valve that the vac line connects to. Mine snapped completely off less than a week ago on the way to work. Talk about a butt puckering experience! The brakes still worked but not nearly as easily as with the boost