can someone explain the brake booster?

[shwartzbewithyou]

I'm wondering how the braking system works for parts like the booster and vacuum pump.  Can someone explain them?  I need to trouble shoot my system.  

When sitting at a light in the summer heat I sometimes lose brake pressure but when the idle is up and I'm driving the pressure comes back.

So I've just never known what the brake booster and vacuum pump do and how they tie into the system.

[QuickTD]

The vacuum pump creates a partial vacuum. This vacuum is applied to both sides of a diaphram inside the brake booster. As you press the brake pedal a valve closes the vacuum port that feeds the rear side of the booster diaphram and air is bled in, this increases the pressure in the on the rear side of the diaphram and drives it forward, applying pressure to the master cylinder pushrod.

 The vacuum pump can move more air at higher engine speeds, so if you have a leak in the vacuum system the pump may be unable to keep up with the leak at idle. That said, the check valve in the hose running to the master cylinder should prevent the loss of vacuum within the booster at idle even if there is a leak in some other part of the vacuum system (other than the booster itself). The vacuum stored in the booster is sufficient for a few applications of the brakes even with the engine off. I see your problem being one of two things, either the booster itself is the source of the leak (more likely) or the vacuum system has a leak somewhere else AND the check valve in the line to the booster is also leaking (less likely?).

[BlackTieTD]

thanks bruce. you reaffirmed some thoughts i had on this. great info as always  

[Master ACiD]

i dont think it matters much if the check valve works or not. on my car i dont even have a check valve and my brakes work great at idle. i can pump the petal as much as i want and i dont lose my vacuum assist.

[QuickTD]

It shouldn't matter if the check valve works. However, if you have a leak elsewhere in the system, a leaking check valve will cause a loss of brake boost at idle as the vacuum is no longer trapped in the booster, it is free to escape to the rest of the system should the pump be unable to keep up. A leaking (or absent) check valve will also cause a total loss of brake boost if the engine stalls, this might be a good reason to replace it...

[Master ACiD]

the vacuum pump its self has a check valve built in. if the engine stalls or something the booster also has a reserve capacity of vacuum inside it, enough for a few pumps on the petal.

for instance, if i start my motor and idle it for a few seconds then shut it off, i still can pump the brakes a few times before they go hard.

[VW Scully]

Hope this doesn't hijack the thread, :oops:  but my brakes are really spongey/squidgey (is that a word?? lol) lately.
What would cause that?? I just replaced pads and rotors last year but all this discussion about brake booster/air in the lines has me thinking.
However, my problem is all the time; not intermittent; brakes are just spongey.
Thanks for any help!  

[LeeG]

VWscully,
spongy is either air in the lines or something bending when you put the brakes on.  

Air: clamp off each rubber hose by carefully squeezing it with a pair of vice grips.  Solid pedal?
yes - have someone release one clamp at a time while you hold down pedal.  it will drop a bit with each one, but you should be able to tell when you get to the spongy one that needs bleeding.
No - there is air in the master or one of the steel lines, bleed the hole system.

bending: usually back drum shoes that are not arced to match the drum and they bend.  when you pull the drum you will see contact areas at one end of the shoes.  if your drums are decent and the shoes from a good source, this shouldnt happen.  You can also sometimes get flexing inthe calipers, it usually means something is hanging up (rusty) like a caliper slide.  I once had a non parallel sided new pad do this, but I dont think it would matter with the A1-A2 caliper design.

[VW Scully]

Thanks LeeG!  

[VelocityConservation]

VW Scully,

The worst case I ever had of spongy brakes turned out to be the rubber brake lines.  I had one that would expand as you pressed the pedal.   If any of yours are more than 7-8 years old I would replace them.

VelocityConservation

[shwartzbewithyou]

Thanks guy's but the more I think about this problem the less likely I think the booster or vaccuum pump would be my problem (now that I know how they work thanks to www.howstuffworks.com, and you).  My issue is that the brake pressure will drop at idle and the pedal will slowly go to the floor.  It seems to only happen at idle after I just stopped but i will have to verify this again.  If the booster was leaking or the vacuum not pumping then my braking power would not be very significant but the pedal would still hold it's height and travel.  So the fact that my pedal is creeping to the floor tells me that the seals in the Master Cylinder may be going (because I can't find a leak in the system.  But again I need to verify if this goes away when the rpm come up.

[LeeG]

If the master is leaking it is either going from one resevoir to the other or leaking out the back of master.
If the level in the 2 resevoirs isnt changing, the try loosening the bolts holding the master onto the booster and pull it forward enough to see if the back of it is all oily with leaked fluid.  On a many cars you can do this with the brake lines still attached, not sure about yours.  Often just cracking the seal between master and booster will let the accumulated fluid dribble out and that confirms the problem.

[shwartzbewithyou]

Could it just be the seal around the piston?  When it compresses the fluid just escapes back into the primary side?  Will that give the same symptom as what you've just explained?

Thanks