Brake Booster Question

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Calipag

Calipag

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We just replaced the brake booster on the 1970 Dart with a fresh 318 and a very (near factory) cam and it is not working. I have never had a new booster that was bad and want to be sure that I have checked everything before sending it back (partgeek).

I did the pedal test and it failed. The pedal stays the same and hard.

I pulled vacuum at the booster with a hand pump and could only get it to 14 psi but it held that vacuum for 5 min. I read that I should pull 20 psi to do the test but it would never go higher that 14-15. When I tap the brake it only drops 5-8 psi at a time. It does not go to directly to zero. I am getting about 15 psi from the engine at idle and it increases with the throttle.

Thank you for any input
 
possibly the pushrod length is too short.
There is a control valve inside the booster that the pushrod has to activate.
If the rod is too short, you get no boost.
If the rod is too long it will activate too early and too hard and put your nose into the windshield.
If the rod is a lot too long, you may break the control valve.

The booster also has to see atmosphere on the firewall side. So if you sealed it, it cannot function. That control valve I mentioned, controls the application of atmospheric air into the firewall side of the diaphragm. It is this air that actually provides the boost. No atmosphere equals no boost.
 
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AJ/FormS said:
possibly the pushrod length is too short.
There is a control valve inside the booster that the pushrod has to activate.
If the rod is too short, you get no boost.
If the rod is too long it will activate too early and too hard and put your nose into the windshield.
If the rod is a lot too long, you may break the control valve.

The booster also has to see atmosphere on the firewall side. So if you sealed it, it cannot function. That control valve I mentioned, controls the application of atmospheric air into the firewall side of the diaphragm. It is this air that actually provides the boost. No atmosphere equals no boost.
Click to expand...

I've read that if the rod is too short you'll have play/slop in the pedal and if its too long the brake will hang or grab. But I could not find anywhere describing what problems I'm having. Have you had the same problem? I'm guessing I should try lengthening the rod but don't want to ruin the booster.
 
Calipag said:
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I pulled vacuum at the booster with a hand pump and could only get it to 14 psi but it held that vacuum for 5 min. I read that I should pull 20 psi to do the test but it would never go higher that 14-15. When I tap the brake it only drops 5-8 psi at a time. It does not go to directly to zero. I am getting about 15 psi from the engine at idle and it increases with the throttle.
Thank you for any input
Click to expand...

It is inches of Mercury - "Hg. Not pressure, negative vacuum. But we knew what you mean. :D

What sort of brakes? Kelsey-Hayes w/ a dual diaphragm booster? Drums? Your vacuum should never go to zero. Any pics?
 
Calipag said:
I've read that if the rod is too short you'll have play/slop in the pedal and if its too long the brake will hang or grab. But I could not find anywhere describing what problems I'm having. Have you had the same problem? I'm guessing I should try lengthening the rod but don't want to ruin the booster.
Click to expand...
These are extreme examples. At small mismatches, the tell-tales are much more subtle.

Wait a sec; you said the pedal is hard from the get-go, right? Ima thinking then that the unit is defective. Either the vacuum you are measuring is not actually accumulating inside the power chamber, or atmosphere is not finding it's way past the control valve.
If vacuum was in there and the control valve was broken,in my experience, the brakes become un-modulatable; it goes from no boost to mega-boost with the slightest pedal application.
 
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Demonic said:
It is inches of Mercury - "Hg. Not pressure, negative vacuum. But we knew what you mean. :D

What sort of brakes? Kelsey-Hayes w/ a dual diaphragm booster? Drums? Your vacuum should never go to zero. Any pics?
Click to expand...

I used a donor 1974 Dart and its a factory front disc car with rear drum. The booster is a 9" booster.

I don't have pics.
 
AJ/FormS said:
These are extreme examples. At small mismatches, the tell-tales are much more subtle.

Wait a sec; you said the pedal is hard from the get-go, right? Ima thinking then that the unit is defective. Either the vacuum you are measuring is not actually accumulating inside the power chamber, or atmosphere is not finding it's way past the control valve.
If vacuum was in there and the control valve was broken,in my experience, the brakes become un-modulatable; it goes from no boost to mega-boost with the slightest pedal application.
Click to expand...

Yep, its hard and stays hard. I never get the power assist.

What I find odd is almost every "test" I find on line says to hand pump 20 Hg and I can not get the booster pumped more that 14-15 Hg. I should have had mine rebuilt but I bought this from PartsGeek and now have to deal with a return!
 
When you say pump, I assume you mean "pull a vacuum", right?
If you can pull a vacuum on it , whatever the number,and it holds it;that only means the diaphragm is intact and nothing else. All the working parts of the booster, are on the atmospheric side.
hydraulic-brake-booster.jpg

55360-004-A413A25E.jpg
 
AJ/FormS said:
When you say pump, I assume you mean "pull a vacuum", right?
If you can pull a vacuum on it , whatever the number,and it holds it;that only means the diaphragm is intact and nothing else. All the working parts of the booster, are on the atmospheric side.View attachment 1715317630
View attachment 1715317631
Click to expand...

Yes, pulling vacuum. I have a shop nearby that is going to test the booster tomorrow on the car. He also said using a hand pump to try and test the booster is like trying to inflate the Goodyear blimp with your mouth. I will let you all know what he ends up telling me.

Thank you
 
WELL! I was on my way home with the Dart. This is the first long drive since we built her and had her aligned. I was going to go to the brake shop today to have the booster tested.
The car drove great on the side streets, alignment was great and the car was running well.
Then I jump on the freeway and get up to 50-55 and everything is good for a while then I get this terrible vibration through the whole car. It shook as if I had a flat, and I actually thought I may be losing the driveshaft. I pulled off the freeway to the nearest parking lot and checked over the car. Everything seemed the way it should be. I convinced myself that it was the road and started on my way again. The car ran fine, steering fine everything seemed good. After driving for awhile it did it again. I pulled to the shoulder and waited and then continued on. I got off the freeway and took side streets home the rest of the way. The car seemed fine on the side street even after getting up to 45-50. It scared me to the point I'm considering throwing in the towel and selling it. This is my son's car and we built it together but I am really over all the trouble we have had with this. Plus I still have my power brake issue.
Why would this shake so badly. I checked all the wheel bearing, driveshaft and the tires are brand new. I think I'm losing my mind.

IMG_5387.JPG


IMG_5388.JPG


IMG_5393.JPG
 
At what speed ,and
what power setting

nice looking car btw

What is a "great alignment"?

The primary go-to is tires, which; the faster you go, the worse it gets.
The secondary, is U-joints and pinion angle.The more power you put into the driveshaft, the higher the pinion climbs, and if it goes nose up, the angles flip from cancelling eachother to combining, and that becomes a bit of a shaker.
Often over looked is the toe-in, coupled with loose steering; the tires load the steering and then it unloads and this goes on and on producing a vibration.
And finally the body itself has a frequency that it oscillates at that is unique to every car right around 58mph (+/- 2mph)on A-bodies as I recall, and that oscillation is aggravated by any other vibration that gets into the chassis.
 
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AJ/FormS said:
At what speed ,and
what power setting

nice looking car btw
Click to expand...
Thank you. We bought it with the crazy strips and was supposed to be address once on the road.

It has happened in 3rd gear around 45-55. But it goes away and then after being in 3rd for a while and a 50ish for a bit it starts to happen. And the shake is through out the car, not just steering but I can see the hood shaking, mirrors etc.
 
Start with checking the tires for slipped or broken cords, or out-of-round; and balance of course.
Then the toe-in.
Then the u-joints.
And finally the pinion angle.

If it was my car, I would go straight to the pinion angle, because
I know my tires are ok, and so on.
The pinion angle has to be "nose-down" relative to the driveshaft, and stay nose down under power. A broken rear spring, or saggy springs, cannot control that with a hi-torque engine, especially with an altered ride-height.
Your car looks a lil low in the back, and that steals caster, which de-stabilizes the front end. So if there is a condition there that already wants to vibrate, the lack of caster will allow it.
 
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AJ/FormS said:
Start with checking the tires for slipped or broken cords, or out-of-round; and balance of course.
Then the toe-in.
Then the u-joints.
And finally the pinion angle.

If it was my car, I would go straight to the pinion angle, because
I know my tires are ok, and so on.
The pinion angle has to be "nose-down" relative to the driveshaft, and stay nose down under power. A broken rear spring, or saggy springs, cannot control that with a hi-torque engine, especially with an altered ride-height.
Your car looks a lil low in the back, and that steals caster, which de-stabilizes the front end. So if there is a condition there that already wants to vibrate, the lack of caster will allow it.
Click to expand...

The rear springs are one of a few things that are not new. I won't be able to work on it this weekertd- Mother in Laws surprise birthday party and we'll be out of town.
Next week I'm going to climb around and under this to be sure I didn't miss something.

Thank you
 
15 inches of vacuum is enough for the power booster to work fine. Mine idles at 13-14 and the brakes work ok. A vacuum is a vacuum, doesn’t matter how you generate it. 15 inches with a hand pump is the same as 15 inches with a compressor.

I have an old compressor from a refrigerator that I use for a vacuum pump, works great. I have used it for about 30 years. Just solder some ac fittings on the lines. Although the pundits say they won’t work, you can use one out of window air conditioners too. Once again, a vacuum is a vacuum.
 
Raise the car off the ground and set it level . Get in and run car through what it did when it was shaking . Di it again to be sure . If it shakes you know it's the back end . If it doesn't you know it's the front .
 
My son pulled the brake booster yesterday and this is what we found. I'm not sure if this may have had something to do with the shake while driving but I think it explains why I have no power assist in the brake system. Sent it back to Parts Geek for a refund today and we are having the original rebuilt by a local shop.

IMG_5395.JPG
 
UPDATE:
So we sent the original brake booster out for rebuild (Karps in Upland Ca. great people) and returned the one we bought from Parts Geek. We also replaced the Master cylinder since it was the only thing we really haven't replaced.
We got the brake booster back yesterday and installed it and the master cylinder last night. I pulled it out of the driveway and did a couple of laps around the block just see how things were and everything seemed fine; the pedal was a little low but we will bleed the brakes again this weekend and I think it should fix that. So I chanced it and took it for a longer drive keeping it at low speeds. Again, everything seemed fine the brakes were working well and the car was driving well. I then decided to get her up to speed and see about the shake........and nothing; all was well. We drove her for about 20 min getting it up to 40-55 mph and NO SHAKE! So at this point I drove her to the freeway and jumped on. I cruised in the slow lane for about 4-5 miles at about 50-60 with no shake and then took her up to 75-80 for another 5-10 miles and it drove very nice. Alignment was nice, the braking was nice and NO SHAKE. We jumped off the freeway and took side streets home and everything was nice and as it should be. She pulls a little to the left when braking and I still want to bleed the brakes again but I think the shake situation was in the braking system. Could a faulty or bad booster apply the brakes enough to grab on its own? My thought is while driving at higher speeds could the engine be increasing vacuum in the booster which would apply the brakes???? Because in our shaking situations with the Parts Geek booster, every time we applied the brake it would go away thus bleeding off the vacuum??? Just a thought. I'm really just happy that the shake is gone and the brakes are almost (the pedal is a little low) where I think they should be.

Thanks to everyone for the support and help! There is no better site in my opinion than this one. The best information, knowledge and most of all understanding of what owners go through. I should add patience for a Chevy guy trying to build his first Mopar!

Thank you,
Matt
 
If the brake booster was dragging the brakes they would heat up and start to shake. Have seen it before with just one sticking caliper causing a lot of vibration.
 
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