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Old 12-27-2007, 09:41 PM   #1
Merlinhaggard
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Default Brake booster test

Is there a way to test a power brake booster?.
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Old 12-27-2007, 09:53 PM   #2
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Originally Posted by Merlinhaggard View Post
Is there a way to test a power brake booster?.

Yep. This is what it should do if it's working properly.

With the engine off:

Pump pedal until it gets hard and won't depress any further.

Start engine while pressing brake pedal.

(Pedal should get soft and stop at the point where your brakes normally lock up at.)

If the pedal never gets hard or if it doesn't get soft, then you have some sort of problem.

Look at and replace if needed the check valve and filter in the vacuum lines.

Also, an always mushy pedal could mean air in the system or a bad master cylinder.
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Old 12-27-2007, 10:02 PM   #3
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Originally Posted by Durango_boy View Post
Yep. This is what it should do if it's working properly.

With the engine off:

Pump pedal until it gets hard and won't depress any further.

Start engine while pressing brake pedal.

(Pedal should get soft and stop at the point where your brakes normally lock up at.)

If the pedal never gets hard or if it doesn't get soft, then you have some sort of problem.

Look at and replace if needed the check valve and filter in the vacuum lines.

Also, an always mushy pedal could mean air in the system or a bad master cylinder.

OR a bad booster.....I proved that one on my own car....

nothing good as Hydro Boost....nothing, not any vacuum assist system that is....
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Old 01-26-2008, 12:32 AM   #4
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Originally Posted by Durango_boy View Post
Yep. This is what it should do if it's working properly.

With the engine off:

Pump pedal until it gets hard and won't depress any further.

Start engine while pressing brake pedal.

(Pedal should get soft and stop at the point where your brakes normally lock up at.)

If the pedal never gets hard or if it doesn't get soft, then you have some sort of problem.

Look at and replace if needed the check valve and filter in the vacuum lines.

Also, an always mushy pedal could mean air in the system or a bad master cylinder.
Okay guys...you may have just solved my problem with brakes and I just want to confirm if you don't mind.


I get a very hard pedal with the car off. Start it and then press the brakes and it goes straight to the floor. Is this right and how sure are you this is a correct method? Sorry to second guess you, but swapping out the booster is not an easy thing if you know what I mean.

2 months now it has been up on jacks


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Old 01-26-2008, 12:42 AM   #5
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is there any way to bench test a power brake booster?

i know the one i currently have is leaking, and ive got another one sitting on a shelf here that im assuming is good but dont know.
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Old 01-26-2008, 07:35 AM   #6
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Originally Posted by 76 sting View Post
Okay guys...you may have just solved my problem with brakes and I just want to confirm if you don't mind.


I get a very hard pedal with the car off. Start it and then press the brakes and it goes straight to the floor. Is this right and how sure are you this is a correct method? Sorry to second guess you, but swapping out the booster is not an easy thing if you know what I mean.

2 months now it has been up on jacks


Your brake pedal should never go to the floor. You've got something else going on such as air in the system. Fix this first and then check the booster. In the mean time, I wouldn't drive the car.
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Old 01-26-2008, 07:42 AM   #7
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Your brake pedal should never go to the floor. You've got something else going on such as air in the system. Fix this first and then check the booster. In the mean time, I wouldn't drive the car.
Sorry I did not do a good job explaing my pre bleeding and brake replacement process so I will be quick.

New everything except the booster. Gravity bleed a solid qt per caliper over the last week. Used the Motive bleeder on each calip while tapping the calip and pulling on the pads to remove any additional internal air. Tried speed bleeders, and the manual 2 man bleeding job.

At this point no matter how much I bleed not a buble of air come out.


This is why I am thinking it is the booster as I have nothing else to lean on.

Does anyone have any other ideas to try?

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Old 01-26-2008, 07:48 AM   #8
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Originally Posted by 76 sting View Post
Sorry I did not do a good job explaing my pre bleeding and brake replacement process so I will be quick.

New everything except the booster. Gravity bleed a solid qt per caliper over the last week. Used the Motive bleeder on each calip while tapping the calip and pulling on the pads to remove any additional internal air. Tried speed bleeders, and the manual 2 man bleeding job.

At this point no matter how much I bleed not a buble of air come out.


This is why I am thinking it is the booster as I have nothing else to lean on.

Does anyone have any other ideas to try?

Does everything new include the master cylinder? If so, did you bench bleed it prior to installation. Again, it sounds like air in the system with your pedal going to the floor.
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Old 01-26-2008, 08:32 AM   #9
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Does everything new include the master cylinder? If so, did you bench bleed it prior to installation. Again, it sounds like air in the system with your pedal going to the floor.
Yes new MC Wilwood Tandem and it has been bench bleed several times.

I kinda agree with the air, but then why do I have a hard as rock pedal with the engine off?

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Old 01-26-2008, 12:13 PM   #10
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The pedal is hard with the engine off because the booster has no vacuum to provide assist; you're basically operating the brake system manually at that point. The master cylinder in a power brake system is not designed to operated manually, and therefore there is high effort required to try to operate the brakes-rock hard pedal. The fact that you pedal drops when you start the engine tells me the booster is fine. If the pedal truly goes to the floor, you have a another problem. Does the car stop?

A booster problem causes a hard pedal and the car is difficult to stop.
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Old 01-26-2008, 12:34 PM   #11
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The pedal is hard with the engine off because the booster has no vacuum to provide assist; you're basically operating the brake system manually at that point. The master cylinder in a power brake system is not designed to operated manually, and therefore there is high effort required to try to operate the brakes-rock hard pedal. The fact that you pedal drops when you start the engine tells me the booster is fine. If the pedal truly goes to the floor, you have a another problem. Does the car stop?

A booster problem causes a hard pedal and the car is difficult to stop.
Ok that makes sense. I was able to get a very small amout of brake out of the system a while ago, but the pedal was just about at the floor before it would just slow down.... eventually. I wanted to get at least a half pedal before taking it for a drive.

Another question if you don't mind. With the car off I can stand on the pedal hard..make that very hard and the pedal will not move more than 1/2". So even if the booster was bad I would just have a rock hard pedal that would require a lot more force to engage the brakes correct?

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Old 01-26-2008, 12:47 PM   #12
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It sounds to me like the actuator rod that runs from the booster to the master cylinder could be out of adjustment. Maybe someone else can chime in here that knows more about this. I've never made the adjustment myself.
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Old 01-26-2008, 12:58 PM   #13
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It sounds to me like the actuator rod that runs from the booster to the master cylinder could be out of adjustment. Maybe someone else can chime in here that knows more about this. I've never made the adjustment myself.
now you can see the delima that I am having It is adjusted with no preload at all, but enough to just touch the MC upon install.

I just remembers another funny thing that happened. My friend and I finnished off the old manual way press the pedal to build pressure and open the bleeder to remove any remaining air. However, a few times as he was pumping the pedal it would go straight to the floor (no pedal pressure) when everything was closed tight. When that happened we let it sit for 5 min came back and the pedal was hard again so we had to do that a few times until all the air was out.

I am so fricken tired of this ****... 3 months now and can't get a pedal to go half way

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Old 01-26-2008, 01:22 PM   #14
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So if the booster was bad I would just have a rock hard pedal that would require a lot more force to engage the brakes correct?

If the booster is bad (providing no assist) the pedal would be hard all the time, and you'd have a sore leg from trying to stop the car. If the pedal drops when you start the car, that indicates to me that the booster is holding a vacuum. Try pulling the check valve from the booster and start the engine (plug the vacuum line); the pedal should remain rock hard as though the engine were off and not supplying vacuum-this would simulate the condition if the booster could not hold a vacuum.
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Old 01-26-2008, 01:24 PM   #15
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Originally Posted by 76 sting View Post
now you can see the delima that I am having It is adjusted with no preload at all, but enough to just touch the MC upon install.

I just remembers another funny thing that happened. My friend and I finnished off the old manual way press the pedal to build pressure and open the bleeder to remove any remaining air. However, a few times as he was pumping the pedal it would go straight to the floor (no pedal pressure) when everything was closed tight. When that happened we let it sit for 5 min came back and the pedal was hard again so we had to do that a few times until all the air was out.

I am so fricken tired of this ****... 3 months now and can't get a pedal to go half way

You must be sucking air into the system when you and your friend are trying to bleed the brakes. Try the gravity bleed again and make sure all lines and hoses are tight. Open up one or more of the bleeder screws at a time and let the fluid drip until you have about a halve cup of the fluid from each caliper and taping them lightly in the process. Be sure to keep the m/c pretty much full. With Vettes, you're supposed to work from the closest caliper from the m/c and work towards the one the furtherest. I went thru this once and the only thing that got all the air out was with the gravity bleed. Good luck and don't give up. You'!l get it.
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Old 01-26-2008, 01:28 PM   #16
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...and pulling on the pads to remove any additional internal air.
What do you mean by this?
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Old 01-26-2008, 01:31 PM   #17
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If the booster is bad (providing no assist) the pedal would be hard all the time, and you'd have a sore leg from trying to stop the car. If the pedal drops when you start the car, that indicates to me that the booster is holding a vacuum. Try pulling the check valve from the booster and start the engine (plug the vacuum line); the pedal should remain rock hard as though the engine were off and not supplying vacuum-this would simulate the condition if the booster could not hold a vacuum.
I will try disconnecting the vac line in a few min to see what happens. I would like to at least rule booster out as the issue and move on.

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Old 01-26-2008, 01:34 PM   #18
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Originally Posted by Jud Chapin View Post
You must be sucking air into the system when you and your friend are trying to bleed the brakes. Try the gravity bleed again and make sure all lines and hoses are tight. Open up one or more of the bleeder screws at a time and let the fluid drip until you have about a halve cup of the fluid from each caliper and taping them lightly in the process. Be sure to keep the m/c pretty much full. With Vettes, you're supposed to work from the closest caliper from the m/c and work towards the one the furtherest. I went thru this once and the only thing that got all the air out was with the gravity bleed. Good luck and don't give up. You'!l get it.
We used the motive bleeder so I do nto think any air got in. The last few air bubbles were removed via manual 2 man job, but we communicated flawlessly on every step.

I am going to gravity bleed them again today and see what happens stay tuned and pray for me.

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Old 01-26-2008, 01:39 PM   #19
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Yes new MC Wilwood Tandem and it has been bench bleed several times.
Is this a factory replacement master cylinder? What is the bore size?
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Old 01-26-2008, 01:39 PM   #20
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What do you mean by this?
I have heard that a lot of air can get trapped in the calip and to tap on them and to pull and or push the pads away from the rotor, which helps remove any remaining trapped air. I did this a few times after I thought all the air was out and it actuallly worked. I was gravity bleeding and I pulled on the outer pad hard and a few more air bubbles came out so I continued around the car and 2 more wheels had air within.

I am heading out there again now to gravity bleed again.

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