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:

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.

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.

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.

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?

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.

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

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.

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.

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!

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.

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...

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.

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.

The procedure with clamping off the hoses is the method they train new mechanics with, they don't have time to guess.
If you haven't done that, then you don't positively know.
Realize it's also going to take a few miles to bed in the new pads, that will affect braking distance.
Going about 10 mph, step very lightly on the pedal and see how far down it goes before you can feel the brakes starting to drag. It should be withing the first inch.

Also check the booster to master rod for proper length.
Lengthen rod until there is a hairline space between booster and master flange, then shorten until it just disappears.

To loosen tough air bubbles another method is to oscillate the lines and calipers while bleeding. Maybe you can borrow your wifes vibrator.:D

As far as the booster R&R, jack up the front of the car as high as you can, kneel beside it, remove the lower a/c vent if an air car, use a 2 foot long 3/8" extension and ratchet with a deep 9/16" socket and universal. Takes less than 10 minutes to remove or install, clip on the clevis is the worst.

OK well this morning i went out and picked up another BRAND NEW master cylinder since im getting alot of people saying thats what my problem is. I bench bled it and installed it and bled the entire system using my Motive power bleeder, and i am still having the same issue. The pedal is firm with the engine off, and goes nearly to the floor with the engine running. I dont know what else to do other then try changine the booster? I am now on my FOURTH master cylinder. Any other thoughts?

Again..imho.. air in system....put some power bleeders on, that will pretty much prevent any air sucked back up for whatever reason... there cheap.
but i had the same symptoms... after replacing everything (my culprit was my proportioning valve) but still had to bleed it over a period of 1 month... a little air here and a little air there,,, finally all gone...

BUT you will always have a mushy peddle with the vacuum booster, if you want a firm peddle there in no option other than Hydroboost,,, and if you are going to pull the VB anyway if you feel its bad (probably not) then I and most all others that have converted will tell you the same... go hydorboost.....

i say air in system too then if your on your 4th master. hope when your bench bleeding master your dont push all the way in until it stops, the cups in there prone to messing up when you do that. gravity bleed couple hours then vacuum bleed a couple quartz threw. guess i got lucky when i replaced all 4 calipers, flex lines and master quick bled all 4 corners and was in business.

When I bleed the master, I just bolt it on the car and pump the pedal instead of using a vise and doing it by hand, I don't think that would make it go too far. I am using a SYNTHETIC Dot 3 fluid, could that be causing any of my problems? Thanks again everybody for ur help.

Have you read this sticky in C3 Tech?
http://forums.corvetteforum.com/c3-t...fo-please.html

that is not bench bleeding..... bad angles in the car to work the air out of the system....... again..... air in the system...

BENCH bleed the mc... and then power bleed for a week....you got air

Here is how you have to bench bleed it and why.
The ports are not at the top of the bore and the angle mount installed in the car makes it worse. If memory serves me correctly the top of the ports are approx 0.060 below, leaves quite an air gap.

Second pic shows a master I cut apart, was in a car with decent brakes for at least a couple years. Notice the corrosion from the air bubble.

Third pic is another way to bleed, moving the master around a bit to remove the air space.


http://i74.photobucket.com/albums/i2...s/Pitting1.jpg

http://i74.photobucket.com/albums/i2...sureMaster.jpg

Here are some other instructions

http://www.filesend.net/download.php...718e5f56411c63

U bE da bOmB noonie!!! :iagree::iagree::iagree::thumbs::cheers:

there is a such thing as dot 3 synthetic? thought only dot 5 was synthetic? anyway that would be the wrong procedure as stated by others for bleeding the master.more than likely your problem.

I would suspect it is a synthetic made like synthetic oil vs using actual oil based products. The DOT 5 is Silicon and is not hygroscopic like regular brake fluid is.

:iagree: or.. got time?...try gravity bleeding...one bleeder at a time for at least 1hr each...furthest to the closest.. MC cap off...keep it topped off and let er drip! :thumbs:

Make sure to fill and cap MC before touching the pedal. :willy: Also make sure p-valve is in good working order. :cheers:

Just wondering if you ever resolved this issue? Did the booster turn out to be bad?

I was having the exact same problem. Replaced the proportioning valve and it did the trick. Was not looking forward to changing the booster.