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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.:cheers:
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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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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. |
Originally Posted by pauldana
(Post 1576241024)
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 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)? |
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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Originally Posted by 509vette
(Post 1576241160)
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.
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? |
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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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. |
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 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. |
Originally Posted by noonie
(Post 1576243270)
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 |
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. |
Originally Posted by mysixtynine
(Post 1576243745)
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. |
Originally Posted by 509vette
(Post 1576243886)
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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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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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 |
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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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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Originally Posted by 509vette
(Post 1576245896)
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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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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