Brake Issue

DCMoney

Well-Known Member
I've been working through an issue with brakes for a few weeks. Brakes have been, vacuumed bleed, pressure bleed, and bleed the old fashion way.

With the car off the pedal is nice and stiff, with it running, edit: (when brakes are applied) the pedal slowly sinks to the floor. Faster after vacuum builds up.

Brakes are not spongy. I'm not loosing any fluid, there are no leaks. My bleeder screws are pointed up. Master cylinder was bench bled.

Things I've tried:

Bleeding till the cows come home.

Replaced master cylinder. Was bench bled.

Check valve on brake booster is working properly.

Pistons are holding the entire time and releasing as soon as you lift off the pedal, so its not a sticking piston.

Plugged both master cylinder ports, pedal does not sink.

Testing the front brakes and plugging only rear brake port, pedal sinks.

Testing the rear brakes and plugging only front brake port, pedal sinks.

Put short hoses feeding the master cylinder to watch the fluid level to make sure the fluid wasn't being displaced between the front and rear ports. After the initial press that dropped the level just a little it didn't move until you lifted off the pedal.

Is it possible to have too much vacuum assistance? When the car idles and you get on the brakes for the first time after a few seconds of it running you get that woosh sound of all the vacuum leaving the booster.

Could it be the booster? I figured it was ruled out with plugging both ports on the master cylinder and the pedal staying firm.

Other than replacing the brake calipers I'm out of ideas, any help would be appreciated.
 
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RichRich

Well-Known Member
When you bled your brake calipers was the bleeder port at the top? if they're installed backwards the bleeder port will be towards the bottom and not allow all the air to evacuate the caliper, just the first thing that comes to mind that wasn't mentioned
 

DCMoney

Well-Known Member
When you bled your brake calipers was the bleeder port at the top? if they're installed backwards the bleeder port will be towards the bottom and not allow all the air to evacuate the caliper, just the first thing that comes to mind that wasn't mentioned
Yep, mentioned in the 3nd paragraph, 3rd sentence.
 

RichRich

Well-Known Member
I am a firm believer in the pressurized method bleed for new systems, I know you said you did it but what were you using to bleed them?
 

Ross

Well-Known Member
When you pump the brake pedal, do the brakes work better?
Just trying to see if you have air in the system...

When I bled my brakes this last week, I had a friend pump the master until it compressed the air, then I broke the bleeder loose on one wheel.
We did this repeatedly until all 4 wheel calipers were bled.
This method worked better for me, than vacuum bleeding, and better than single pump pressure bleeding.
 

ctuinstra

Well-Known Member
I am completely against vacuum bleeding and recommend pressure bleeding instead. Vacuum can suck air in to the system. The slightest leak anywhere in the system can allow air in. I have never been able to get a good enough seal on the bleeder hose or the threads of the bleeder screw to stop the sucking of air in while bleeding and there for always see air in the fluid (which you think is in the line, but actually just the bleeder screw).

You may want to pull off the calipers and see if you can see weeping around the pistons or under the dust seal. One or more may be leaking down. If you have old rubber brake hoses, you can clamp them off one by one to see if you can find which one is leaking, but that's not good for the hoses.
 

DCMoney

Well-Known Member
When you pump the brake pedal, do the brakes work better?
Just trying to see if you have air in the system...

When I bled my brakes this last week, I had a friend pump the master until it compressed the air, then I broke the bleeder loose on one wheel.
We did this repeatedly until all 4 wheel calipers were bled.
This method worked better for me, than vacuum bleeding, and better than single pump pressure bleeding.
Typically 3-4 pumps with the car running and they get stiff, until vacuum builds back up (few seconds later) pedal then starts to sink. Don't touch the brakes for 5+ seconds pedal goes 3/4 of the way to the floor on first press, car still stops. Disconnect vacuum assistance, brakes work as expected.

I am completely against vacuum bleeding and recommend pressure bleeding instead. Vacuum can suck air in to the system. The slightest leak anywhere in the system can allow air in. I have never been able to get a good enough seal on the bleeder hose or the threads of the bleeder screw to stop the sucking of air in while bleeding and there for always see air in the fluid (which you think is in the line, but actually just the bleeder screw).

You may want to pull off the calipers and see if you can see weeping around the pistons or under the dust seal. One or more may be leaking down. If you have old rubber brake hoses, you can clamp them off one by one to see if you can find which one is leaking, but that's not good for the hoses.
Disassembled one front caliper today, no signs of leaking. Didn't have time to dissemble the other.

I have the rubber hoses that came with the kit didn't really want to swap them out with the SS lines, might swap the lines out or plug the line at the tee and hopefully narrow down the bad calipers. Anyone know what size the tees are? 3/8-24 inverted double flair?
 

Desert Sasqwatch

Well-Known Member
DC, I think you have an issue with the brake booster. Vacuum leak, broken spring or something, I am just not sure what, because I have never heard of this issue that was not related to the calipers or air in the brake line. But from your first narrative, it appears you had covered everything except the booster. Just my 2 cent worth.....
 

DCMoney

Well-Known Member
DC, I think you have an issue with the brake booster. Vacuum leak, broken spring or something, I am just not sure what, because I have never heard of this issue that was not related to the calipers or air in the brake line. But from your first narrative, it appears you had covered everything except the booster. Just my 2 cent worth.....
With the master cylinder ports plugged and the pedal not sinking stopped me from thinking it was the booster.
 

Ross

Well-Known Member
Typically 3-4 pumps with the car running and they get stiff, until vacuum builds back up (few seconds later) pedal then starts to sink. Don't touch the brakes for 5+ seconds pedal goes 3/4 of the way to the floor on first press, car still stops. Disconnect vacuum assistance, brakes work as expected.
This sounds like air in the brake system to me.
 

DCMoney

Well-Known Member
Bad master...... And maybe bad return spring, which could be internal.
Brand new master cylinder, old one which worked on the donor did same thing. To test the internal seals of the MC I put two short pieces of hose on the ports, stepped on the brakes to see if fluid was being transferred from one port to the other etc. level went up and down in both equally. If seals were bad I would get pedal sink to the floor without boost assistance as well.


My plugs arrived Monday so I can cap off at each of the calipers to attempt to start narrowing down the problem. I have a new brake booster to test as well in case it's something internal on the booster.

This thread is a good read, guy is having almost identical issues I am, he never figured it out, only thing he didn't do was replace all the hard lines.
Years later he responded saying the brakes got better, believe it was air trapped somewhere that slowly worked its way out.

https://forums.tdiclub.com/showthread.php?t=366442

If the booster doesn't change anything and plugging before the calipers doesn't change anything, then it has to be air in the lines. Being that I had blockage in my lines that had to be cleared with welding wire to get fluid to flow makes me think there could still be something causing air to be trapped in the line. I'm planning on bringing works brake line kit home with me this weekend so I can make new lines if needed.
 

George

Well-Known Member
Some times the brake pads need to be well seated on to the rotor. This can only be done by driving in and letting the pads wear to conform to the rotor and the angle of how the caliper is mounted. If it fells save to drive take it out and work on some high speed stops.

Brad
 

Lonny

Administrator
Staff member
I searched and found this on the web.

If the brakes are always spongy, it is air bubbles. If the brakes are only spongy when hot, it is moisture. Pedal feel is always softer until the pads have bedded into the rotors properly. Make sure that you follow a proper bed-in procedure before deciding that you have a problem.Sep 22, 2015

Brad may be correct.

If the pads are not wear matched evenly to the rotors ( bed-in ) your pedal will get firm and then your engine vacuum will start to normalize with the booster which will increase the line pressure forcing the pads to flatten against the rotor. The pedal will continue to move down until the pads can't move anymore and the booster is at maximum vacuum.

Doing a bed-in procedure should make the pedal feel better.

If the pedal just keeps on going down I would definitely not drive it.

This is from our GM Cobalt service manuals.

Brake Pad and Rotor Burnishing
Warning: Road test a vehicle under safe conditions
and while obeying all traffic laws. Do not attempt
any maneuvers that could jeopardize vehicle
control. Failure to adhere to these precautions
could lead to serious personal injury and vehicle
damage.

Burnishing the brake pads and brake rotors is
necessary in order to ensure that the braking surfaces
are properly prepared after service has been
performed on the disc brake system.

This procedure should be performed whenever the
disc brake rotors have been refinished or replaced,
and/or whenever the disc brake pads have been
Replaced.

1. Select a smooth road with little or no traffic,

2. Accelerate the vehicle to 48 km/h (30 mph).

Note: Use care to avoid overheating the brakes while
performing this step.

3. Using moderate to firm pressure, apply the brakes
to bring the vehicle to a stop. Do not allow the
brakes to lock.

4. Repeat steps 2 and 3 until approximately 20 stops
have been completed. Allow sufficient cooling
periods between stops in order to properly burnish
the brake pads and rotors.
 

Torchandregdoc

Well-Known Member
Brand new master cylinder, old one which worked on the donor did same thing. To test the internal seals of the MC I put two short pieces of hose on the ports, stepped on the brakes to see if fluid was being transferred from one port to the other etc. level went up and down in both equally. If seals were bad I would get pedal sink to the floor without boost assistance as well.


My plugs arrived Monday so I can cap off at each of the calipers to attempt to start narrowing down the problem. I have a new brake booster to test as well in case it's something internal on the booster.

This thread is a good read, guy is having almost identical issues I am, he never figured it out, only thing he didn't do was replace all the hard lines.
Years later he responded saying the brakes got better, believe it was air trapped somewhere that slowly worked its way out.

https://forums.tdiclub.com/showthread.php?t=366442

If the booster doesn't change anything and plugging before the calipers doesn't change anything, then it has to be air in the lines. Being that I had blockage in my lines that had to be cleared with welding wire to get fluid to flow makes me think there could still be something causing air to be trapped in the line. I'm planning on bringing works brake line kit home with me this weekend so I can make new lines if needed.
I would have put money on the master. Always learning.
 

Briann1177

Well-Known Member
If you have a master cylinder bore diameter of 1" (.785 sq in) and a caliper piston diameter of 1.5" (1.77 sq in), the brake caliper piston will move 1" for every 2.25" of pedal/master cylinder travel. 1" of travel for the caliper piston is a huge amount even if the brake pads aren't seated correctly.
 

DCMoney

Well-Known Member
Mother F'er, swapped the brake booster, pedal doesn't sink to the floor.

My dad thinks with plugging the ports on the MC, this didn't allow the rod in the booster to move far enough to point out something wrong with the booster. Oh well its over with, what a pain.
 
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