brake booster question

[mechted]

been having some weird behavoir for my 78's brake booster lately, occationally it will fail to boost, pedal just drops and i get manual brakes only... 95% of the time its fine, and it seems to happen mainly when cold (though it has happened a few other times)
ive done the "pump test" a couple times (pump up brakes with engine off, then hold brake as you start) and it still held a firm pedal... so that makes me think 1) im doing it wrong or 2) check valve
any ideas?
and am i doing the test correct?

btw, this is a 78 C20 with a 350

 

[dyeager535]

Brake booster test is to start the vehicle, don't touch brakes, let it idle for a bit (I'm sure a few seconds is enough) turn it off, let it sit for 45 minutes, go back out, don't start vehicle, apply brakes.

You should get two full power assists before the brakes starts to get hard to push.

 

[R72K5]

45 is waaaaay too long,

but yeah jsut start it up let it run a coupel seconds and shut it off

and see if it lets you do a couple of pushes before running out of vacuum

pedal drop would be master cylidner failure,

bad booster would make for extremely hard ot push pedal and little travel, IE you would need ot stand on it both feet to push the pedal in order to slow the behicle up


good luck

 

[dyeager535]

The longer you wait the more likely it is to notice a small leak. A couple of seconds waiting won't indicate a small leak with the volume of the booster.

 

[mechted]

well, you were right, changed the mastercylinder today and now brake pedal is nice and firm thanks guys

 

[R72K5]

glad to see its fixed