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Old 12-17-2010, 09:12 AM   #1
509vette
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Default Do i have a bad brake booster????

Hello everybody, I got a 73 with power brakes, I have replaced all four calipers with remans, brand new hard and rubber lines, brand new master cylinder and proportioning valve. The only thing left that I dident replace is the brake booster. My problem is I have bled the system 3 times in a row and have no air in it anywhere, it has a hard pedal with the engine off, but as soon as I fire up the engine, when I hit the pedal it goes almost to the floor with maybe an inch to spare, the car will still stop, but very very poorly. Could this be a bad booster? Thanks for the help fellas.
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Old 12-17-2010, 09:23 AM   #2
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start engine, turn engine off. then pull vacuum line off booster it is supposed to make a noise relieving vacuum. if no noise then there must be a leak. but your describing something else i think. if pedal goes to floor then i suspect master first then work from there. o ya i have had many many brand new bad masters.
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Old 12-17-2010, 09:33 AM   #3
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Bleed bleed and bleed some more.... you almost have to have a power bleeder for these cars..... when i bleed, i bleed almost a gallon of brake fluid through before all the air was gone.... yea, a real full gallon. and for me without a power bleeder, forget it.

Also, even after you get them perfect(everything 100%), they will always have a slight mushy feel... always... UNLESS you put on a HydroBoost.... then and only then will you have brakes that are out of this world.... a perfect linear feel.... no, none, nata zippo mush in the peddle.... but thats only after a hydroboost conversion.
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Old 12-17-2010, 09:45 AM   #4
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Originally Posted by pauldana View Post
Bleed bleed and bleed some more.... you almost have to have a power bleeder for these cars..... when i bleed, i bleed almost a gallon of brake fluid through before all the air was gone.... yea, a real full gallon. and for me without a power bleeder, forget it.

Also, even after you get them perfect(everything 100%), they will always have a slight mushy feel... always... UNLESS you put on a HydroBoost.... then and only then will you have brakes that are out of this world.... a perfect linear feel.... no, none, nata zippo mush in the peddle.... but thats only after a hydroboost conversion.
I have bled it both the old fashioned way with two people and I have used the Motive power bleeder clamped to the MC. Went through about two thirds of a gallin of dot 3 with no major improvment. I don't know what else to do. With the booster disconnected from the vacuum line the pedal stays firm, like manual brakes.
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Old 12-17-2010, 10:39 AM   #5
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I had a similar problem a few years back. Rebuilt master cylinder, new lines, etc. I gave up and sent it to a shop. Turned out the rebuilt master cylinder was no good. No amount of bleeding helped.

Could be air trapped in the lines somewhere as well. Maybe pressure bleed to get a continuous flow, enough to move an air bubble along thru the system?

The brake booster will make the brakes feel harder when there's no vacuum. If the pedal goes nearly to the floor with vacuum, seems the booster is doing its job.

Are all the pads seated well? No binding? No cracked calipers (never seen one but thinking something may be flexxing under great pressure)?
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Old 12-17-2010, 10:41 AM   #6
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I had the same feel. turns out I had both a bad power booster (pull the vacuum line off after turning the engine off and you should hear a hiss if it is OK) and a bad new master cylinder.
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Old 12-17-2010, 10:44 AM   #7
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Quote:
Originally Posted by 509vette View Post
I have bled it both the old fashioned way with two people and I have used the Motive power bleeder clamped to the MC. Went through about two thirds of a gallin of dot 3 with no major improvment. I don't know what else to do. With the booster disconnected from the vacuum line the pedal stays firm, like manual brakes.
Mine did the same thing..... you still have air somewhere.... did you bench bleed the M/C? before you installed it in the car? if not you will probably have to remove it, bench bleed it and re-bleed the entire system once again.
once all the air is gone, you should be fine, but they will never feel really firm, but yes, your peddle is going to far down, you do have air in the system still most likely. Especially since you replaced everything already.

does your brake light come on when the peddle goes to the floor?
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Old 12-17-2010, 11:39 AM   #8
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If you disconnect the booster and the brakes work well, but are just harder to push the pedal. Then I would say you do not need to bleed more. When I found out my booster was bad, disconnecting it resolved the issues I was having with the car idling properly as well.
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Old 12-17-2010, 12:03 PM   #9
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The proper way to test the booster is this: Hold foot on brake pedal with engine off. With brake pedal firmly depressed, start engine. If the pedal sinks when you start the engine, then the booster is good.

If the booster passes the above check then your problem is elsewhere. Most of us here on the the forum use Motive bleeders to pressure bleed from top down. That being said, you still should bench-bleed the master before installing.
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Old 12-17-2010, 12:57 PM   #10
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First of all, measure the pedal movement from rest with the engine off.
You should have no more the 1" pedal travel to firm pedal.
With the engine running, you should have no more than 1/2 the distance to the floor.

A booster can also go bad by depressing the pedal too far to the floor, and if that is the case, then the phenolic inside is cracked or the air valve and control valve inside are not working properly. This is pretty rare.

Here's how to test the hydraulics

Quote:
How to diagnose a sinking, low or spongy brake pedal

1. Begin by isolating the master cylinder. Remove the lines from the
master cylinder and insert blocking plugs into the ports. Press on
the brake pedal. If the pedal is firm and hard, the master cylinder
is good. If the pedal still drops, either air is in the unit or it is
bypassing internally. Bleed the unit or replace as necessary.
Reconnect and bleed the system.

2. With a properly bled master cylinder test the brake hoses. Using
a proper tool, carefully pinch off all the rubber brake hoses as
close to the master cylinder as possible. Release one brake hose
at a time. Press the brake pedal. If the brake pedal is firm and
hard, move to the next circuit. Repeat until the circuit containing
the defective component is located (the brake pedal will be soft
or sinking). Replace defective components as necessary.

3. Flush the system thoroughly with fresh brake fluid from a sealed
container according to the manufacturer’s service procedures.
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Old 12-17-2010, 01:47 PM   #11
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Quote:
Originally Posted by noonie View Post
First of all, measure the pedal movement from rest with the engine off.
You should have no more the 1" pedal travel to firm pedal.
With the engine running, you should have no more than 1/2 the distance to the floor.

A booster can also go bad by depressing the pedal too far to the floor, and if that is the case, then the phenolic inside is cracked or the air valve and control valve inside are not working properly. This is pretty rare.

Here's how to test the hydraulics
Im pretty certain that the master cylinder is good because with the engine off I get a solid pedal, moves only very slightly, and if I hold pressure on the pedal it stays firm and dosent drop.
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Old 12-17-2010, 01:49 PM   #12
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Most likely air in the rear calipers.

Best is to bleed with a motive bleeder (of course doing one wheel at a time). Make sure you crack both bleeders on the rear calipers they seem to trap air more than the fronts due to the way the caliper is positioned.

Also tap on the caliper with a rubber mallet to knock the bubbles of air off the inside of the castings of the caliper.
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Old 12-17-2010, 02:05 PM   #13
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Originally Posted by mysixtynine View Post
Most likely air in the rear calipers.

Best is to bleed with a motive bleeder (of course doing one wheel at a time). Make sure you crack both bleeders on the rear calipers they seem to trap air more than the fronts due to the way the caliper is positioned.

Also tap on the caliper with a rubber mallet to knock the bubbles of air off the inside of the castings of the caliper.
I have done that exact prodecure a few tims already, I smacked all four calipers with a rubber mallet as I opend the bleeders, I got some bubbles then I got pure fluid, then I bled the system two more times after getting pure fluid just to make sure there was no air left in the system.
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Old 12-17-2010, 02:42 PM   #14
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Quote:
Originally Posted by 509vette View Post
I have done that exact prodecure a few tims already, I smacked all four calipers with a rubber mallet as I opend the bleeders, I got some bubbles then I got pure fluid, then I bled the system two more times after getting pure fluid just to make sure there was no air left in the system.
I had to bleed mine on at least 4 separate occasions, i would bleed them, drive her for a while (brakes were so-so but did stop the car) bleed again, improvement, drive... bleed again, improvement... etc... but after getting all the air out finally... she stopped good.... then put in the HydorBoost, now she stops GREAT!
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Old 12-17-2010, 04:06 PM   #15
509vette
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so whats the majority vote? Am i wasting my time and money by replacing the booster? Should i be looking elsewhere for the problem? What does everybody think? Thanks again for the help fellas.
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Old 12-17-2010, 04:35 PM   #16
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As has been said a few times here: your booster is doing what it's supposed to do;
there is air left in the system, or you have a bad master.....
Cor
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Old 12-17-2010, 05:13 PM   #17
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Up to you. We all have our own opinions as to what it could be based on our experiences. I will say replacing the booster can be a bear. Getting that top left nut off is a challenge. A mystery how GM ever mass produced this car...
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Old 12-17-2010, 05:46 PM   #18
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ok the real question is, is it possible for there to be something wrong with the booster that would be causing the problem im having?
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Old 12-17-2010, 06:04 PM   #19
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Quote:
Originally Posted by 509vette View Post
ok the real question is, is it possible for there to be something wrong with the booster that would be causing the problem im having?
All I can offer is disconnect the booster for a week or two and drive the car without it connected. There could be something odd the booster is doing. Driving without the booster connected will be like having manual brakes. It will be hard to stop, but if there is an air issue it will still show up after driving some. If you have not issues it could be something odd with the booster providing too much power assist.
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Old 12-17-2010, 06:29 PM   #20
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In my experience, a bad booster gives you a hard pedal, not a soft one. You still have air some where. This worked for me, may or may not work for you, pump the brake pedal voilently 3 or 4 times and hold it down while the bleeder is opened. Sometimes this will move the air through to the bleeder. Do this several times per caliper and see if you get more air.
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Old 12-17-2010, 06:29 PM
 
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