Master cylinder or brake booster?

[LUMINATOR]

Hey guys ! I got a 1990 Lumina, base model with a spongy brake petal. I bled all the calipers out and can't figure out why my pedal goes soft when I start the car up. There are no brake leaks around the car also . I can pump up the brakes when the car is not running and have a nice firm petal, keep pressure on it, start the car and the petal sinks.

 

If you guys now of anyway to check the master cylinder or brake booster I would really appreciate it!

[Andrew]

there is a tool you can rent to test your booster, see if it holds vacuum. Real easy to use.

 

and dont make 2 threads for the same issue.

Edited April 14, 2013 by Andrew

[LUMINATOR]

Alright thanks, I will look into that!

 

Sorry about that, I'm new to the forum:confused:!

Edited April 14, 2013 by LUMINATOR
spelling error

[AL]

When you start the car, it creates vacuum inside the brake booster which assists in helping you push the pedal, without vacuum, you have no assistance as it has no vacuum to it, so this is perfectly normal for every car

[RobertISaar]

When you start the car, it creates vacuum inside the brake booster which assists in helping you push the pedal, without vacuum, you have no assistance as it has no vacuum to it, so this is perfectly normal for every car

 

and to add to the complexity to make it a bit more real-life, there's also the fact that the booster has a decent sized resivoir in it, so it allows more than a single pump of the brakes before all assist is lost. there's also the check valve on the vacuum line going to the booster so that when there is vacuum in the booster, it stays there when vacuum from the engine is no longer present.