Brake Booster issues

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afbill

afbill

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We have gone through 5 brake boosters in the last year. Sometimes they'll last a few months, sometimes a few hours. Anyone else having this type of issue?

We are running an external vacuum pump set at 20 psi. Could this be too much and blowing out the diaphram? Frankly, I don't know how these boosters work. It will hold vacuum and then when you press the pedal you can hear a pressure release and the pump kicks back on.
 
20psi? Please explain psi?
The booster is supposed to be in a state of vacuum, not pressure.

When you step on the pedal, you are opening the atmospheric valve which introduces 14.7 psi atmospheric pressure into the firewall side of the booster. Since this side of the chamber is sealed from the radiator side, which is under vacuum, the diaphragm moves towards the radiator, and applies the brakes, in proportion to how much atmospheric pressure your foot is introducing; and in addition to how hard you are pressing.... if the pushrod is correctly adjusted.
There should be no migration of air or vacuum, between the chambers.
Yes the vacuum in the vacuum chamber will fall, because the chamber is changing volume, as the diaphragm moves towards the radiator. That chamber normally does not require more than 11 to 12 inches of vacuum to make it work.
If your 20 psi, is actually 20 inches vacuum, it should not hurt the diaphragm, but is far in excess of the minimum requirement, and might make the device overly sensitive.
Hissing would be normal AFTER the pedal is released, as the pressure chamber has to be emptied in preparation for the next application. I think the pressure dumps on the firewall side, right next to it, and usually downwards, sometimes sideways.
As far as the minimum vacuum is concerned; I ran the 292/292/108 cam with a single diaphragm booster out of a 73 Dart318 and it worked fine...... even down to 9inches of idle-vacuum. However, this was on a manual trans car with 3.55s. With an MT the driver usually downshifts as he is coming to a stop, so there is always more than say 11 in the chamber. At first start-up in the AM, a couple of blips of the throttle charges the chamber, and by the time I have backed out of my carport, she's good to go.
If your pushrod adjustment is too long, you may be apt to break the control valve on a panick-stop.
 
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I said PSI but I meant inches of mercury...PSI is just easier. I figured it was known I was talking about vacuum so it would be assumed I meant Hg".

We've never messed with the linkage. That has to be the problem.

Also, thank you for the explanation of how it works, it all makes sense now
 
How do you diagnose "fail"? To test when you have the booster out, just connect the vac hose from another car that is running. When you push in the pedal rod, the output rod should follow w/o requiring much effort to push. It might require you pushing on the output rod to return, as the MC spring does (forgot). You will hear air leak in as it moves, but should be no noticeable hissing when stationary. Being an engineer, stating any measurement w/o units is negligent, and stating the wrong units is a major error. A probe crashed on Mars ~15 yrs ago because NASA assumed a contractor's computer thrust calculation values were in "lbf", when actually "N". Both parties were grossly negligent to ASSume.
 
The maximum vacuum/ pressure you can have is atmospheric, which is 14.7 psi or the equivalent vacuum, 29.9 whatever....... at sea level. Those boosters should have PLENTY of overhead for that.

In fact my recently sorrowfully bought 04 Sierra has a factory electric vacuum booster pump

I'd say either you got these from the same lame rebuilder either doing something wrong or using wrong parts, or you somehow have the wrong pushrod or adjustment, or wrong master (pushrod stroke) and are somehow bottoming something, destroying the valving
 
BillGrissom said:
How do you diagnose "fail"? To test when you have the booster out, just connect the vac hose from another car that is running. When you push in the pedal rod, the output rod should follow w/o requiring much effort to push. It might require you pushing on the output rod to return, as the MC spring does (forgot). You will hear air leak in as it moves, but should be no noticeable hissing when stationary. Being an engineer, stating any measurement w/o units is negligent, and stating the wrong units is a major error. A probe crashed on Mars ~15 yrs ago because NASA assumed a contractor's computer thrust calculation values were in "lbf", when actually "N". Both parties were grossly negligent to ASSume.
Click to expand...
I remember that Mars mission. We're talking about a brake booster not a Mars mission. relax. I have a EE degree and am an aircraft mechanic, I'm no rocket scientist. I diagnose a "fail" as 'unit worked then stopped working'. It "failed" to work. The reason I have troubleshot the problem to the booster is because when the booster is replaced with a known good unit the brakes work correctly. At some point the brake pedal feels like you're pushing against a rock, there's no boost. Replace the booster, brakes work fine again, eventually I get the same results. The problem follows the booster. There is no hissing when stationary and the vacuum side holds a vacuum, which tells me the diaphragm is good. The problem has to be on the ambient pressure side. I believe we have too much throw on the pedal side and are breaking something in the booster. We're gonna tear one apart ( a bad one) and see what is broken. Sorry to have not been technical enough for you.
 
67Dart273 said:
The maximum vacuum/ pressure you can have is atmospheric, which is 14.7 psi or the equivalent vacuum, 29.9 whatever....... at sea level. Those boosters should have PLENTY of overhead for that.

In fact my recently sorrowfully bought 04 Sierra has a factory electric vacuum booster pump

I'd say either you got these from the same lame rebuilder either doing something wrong or using wrong parts, or you somehow have the wrong pushrod or adjustment, or wrong master (pushrod stroke) and are somehow bottoming something, destroying the valving
Click to expand...


We're about to look at the stroke, frankly we've NEVER looked at that. I'm confident that's where the problem is. Also, I've heard of crappy rebuilt boosters too. That may be part of the problem also but I really think something is out of adjustment on the car too.
 
afbill said:
We're about to look at the stroke, frankly we've NEVER looked at that. I'm confident that's where the problem is. Also, I've heard of crappy rebuilt boosters too. That may be part of the problem also but I really think something is out of adjustment on the car too.
Click to expand...
What was the final outcome of your issue? Thanks
 
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