No Power Brakes

askiles

Well-Known Member
So, perhaps this is a common issue, or at least one easily remedied. When I have the vacuum line from the engine connected to the port on the brake booster, my brake pedal stays glued to the floor, and won’t come back up. Basically nearly locking all of the brakes. When I disconnect the vacuum hose I have perfectly good manual brakes that work excellent. They just require a bit of heave-ho to get em there. What could this be?? Bad brake booster? It worked perfectly fine in the Cobalt prior to removal. I did swap out the master cylinder. Could it have anything to do with that? Thanks for the help guys!
 

ctuinstra

Well-Known Member
There may be something wrong with the brake boost or the alignment. The vacuum should not be pressing the pedal down until you press it some to open the ports to allow vacuum. My understanding is the poppet valve opens and allows atmospheric pressure behind the bladder and vacuum in the front thus pressing the pedal down. There could be an alignment issue between the rod and the new MC causing it to "stay pressed" some.
 

askiles

Well-Known Member
There may be something wrong with the brake boost or the alignment. The vacuum should not be pressing the pedal down until you press it some to open the ports to allow vacuum. My understanding is the poppet valve opens and allows atmospheric pressure behind the bladder and vacuum in the front thus pressing the pedal down. There could be an alignment issue between the rod and the new MC causing it to "stay pressed" some.
Do you think if there was an alignment issue with the rod it would still do it in “manual mode”? It works great in manual. It’s almost like the vacuum is causing it to stay depressed.
 

JSATX

Well-Known Member
What’s strange is it didn’t do it in the cobalt. I just chopped up my booster to see how it works, there’s several different springs in there to get it to return, maybe one of them broke?
 

ctuinstra

Well-Known Member
The vacuum is what helps to press the pedal down, so yes it could help hold the pedal down. Without any vacuum, the booster is nothing more than a rod so it will always work in manual mode (without any assist). When the pedal is NOT pressed, there should be vacuum on both sides of the diaphragm (which is a neutral pressure) so therefore it should not be pressing on the rod.

But I would not think that the vacuum alone would be enough to press the MC. Do you have to press it down and it stays down or does it go down as soon as you apply vacuum?
 

askiles

Well-Known Member
The vacuum is what helps to press the pedal down, so yes it could help hold the pedal down. Without any vacuum, the booster is nothing more than a rod so it will always work in manual mode (without any assist). When the pedal is NOT pressed, there should be vacuum on both sides of the diaphragm (which is a neutral pressure) so therefore it should not be pressing on the rod.

But I would not think that the vacuum alone would be enough to press the MC. Do you have to press it down and it stays down or does it go down as soon as you apply vacuum?
It stays down after you press the brakes. It does not go to the floor by itself.
 

askiles

Well-Known Member
What’s strange is it didn’t do it in the cobalt. I just chopped up my booster to see how it works, there’s several different springs in there to get it to return, maybe one of them broke?
Agreed. That is the weird part. Perhaps a return spring is broke, BUT it does return when there is no vacuum applied. So, maybe not?!!
 

JSATX

Well-Known Member
Agreed. That is the weird part. Perhaps a return spring is broke, BUT it does return when there is no vacuum applied. So, maybe not?!!
Well that’s not as surprising because the fluid in the MC is pushing the pedal back up. Are the brakes well bled?
 

ctuinstra

Well-Known Member
I'll bet there is something wrong with the poppet valve in the booster not returning to its position and allowing vacuum to build back up on the back side of the diaphragm (to create neutral pressure).
 

ctuinstra

Well-Known Member
Well that’s not as surprising because the fluid in the MC is pushing the pedal back up. Are the brakes well bled?
That's a good point also. You shouldn't really get them to go to the floor, manual or powered.
 

askiles

Well-Known Member
Well that’s not as surprising because the fluid in the MC is pushing the pedal back up. Are the brakes well bled?
I thought that would have been the issue as well, so I re-bled the brakes all the way around today several times, and it feels solid. Running solid fluid out, no air.
 

askiles

Well-Known Member
Crisis averted!! So it was an alignment issue of the pushrod. Where it sat on the brake pedal arm was just slightly askew, making the pushrod hang up. Aligned that better, and now it works beautifully.
 
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