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As some of you might know have had some issues with my 1993 3.1 Corsicas brakes. Originally found out in inspection that rear brakes were too weak (braking power 0.7-0-8KN)... we use kilonewtons here in Europe.

1. Did rear brakes job but that didnt help, brakes were still too weak (could even spin rear tires by hand when someone press brake). Looked for leaks, found none.

2. Replaced master cylinder with the abs module, took part from scrap car. This is where the fun began. Bleeded the system normally. Now there was more power on the rear tires (couldnt spin them by hand anymore while brake is applied).

3. Bleeded more, because during first bleeding was very busy and didnt bleed the wheels properly, only master cylinder. Ok, went to test drive and tested how the car was stopping, used abs also since it was wet roads. After test drive wanted to make sure brakes are still good and tested if i can spin rear tires while brakes applied. FAIL. Rear tires spinning again by hand.

4. Bled the system again. Did step 3 again but didnt use abs while test driving. SUCCESS. Couldnt spin rear tires when brake applied.

5. Bled the system again. Did step 3 again and USED ABS. FAIL. Rear tires spinning again while brake applied.

6. My conclusion: abs motor/pump assembly has now got air from somewhere. Propably during second time bleeding, when managed accidentally to run reservoir empty and lot of air entered the system. Now when i use abs brakes, air from abs assembly gets to tires. If i bleed the system and i dont use abs, i dont have any issues.

7. Have tried to bleed abs system according to couple of guides. https://www.aa1car.com/library/abs_bleeding.htm and https://www.autozone.com/repairguides/G ... 5280068379

8. None of these bleeding made success. I dont have tech 1 so used the other way; starting car and driving it above 5mph. No success.

9. What i have noticed, when I am bleeding master cylider from the front (or rear) bleeder screw, from both screws I keep getting air forever. Honestly, I have put liters of fluid (and hours of time) though the master cylinder, always air keeps coming. Tiny little bubbles, no big ones.

10. Question is, what next? Is this mc bad too? Where is the air coming to mc bleeder screws??? Maybe abs motor is hogging the air from somewhere? I think I need to look for one more parts car master cylinder and make sure this time air doesnt get to abs section......

All help is appreciated. Car has now been sitting for 6 months due to this problem (cannot pass inspection brake test because of weak rear brakes).

I have seen air getting sucked into the threads on the bleeders too. At both the master and the wheels.
But you might have air in the MC now. If you don't have a MC bleeder kit, get one and make sure it has the fittings to fit your brake line output fittings.
They say it's best to bench bleed the master cylinder because it's mounted on an angle in the car and air gets trapped. I have done mine on the car and the brakes seem to work fine.
I did bleed mine on the bench without the ABS unit on it. Then I assembled and installed it on the car and used the ABS bleeders with a pressurized bleeder tank. But manual bleeding should work OK too. I'd then crack the brake lines a bit to let the air out of them and tighten as the fluid is coming out.
Then bleed the wheel cylinders and calipers.
On the rear, did you adjust the new shoes to contact the drum? Self adjusters might not be working. I usually crank the adjuster until I get some interference when putting on the drums. But I want the drums to have only a little drag when I rotate them. If you don't adjust them up, you'd have to drive the car back and forth while the car stomping on the brakes to get them to adjust up enough. Another thing to check is your parking brake cable. Is it released all the way? If not, it might make your shoes contact only part of the shoe and limit the braking force.
I don't know what you mean, brake without abs. Did you run the lines direct into the MC somehow?
I had heard about a scan tool to set the ABS too years ago. I've since found no scan tool is needed to bleed the system as long as there is nothing wrong with the ABS system. I hope this helps.

ifixalot wrote:I have seen air getting sucked into the threads on the bleeders too. At both the master and the wheels.
But you might have air in the MC now. If you don't have a MC bleeder kit, get one and make sure it has the fittings to fit your brake line output fittings.
They say it's best to bench bleed the master cylinder because it's mounted on an angle in the car and air gets trapped. I have done mine on the car and the brakes seem to work fine.
I did bleed mine on the bench without the ABS unit on it. Then I assembled and installed it on the car and used the ABS bleeders with a pressurized bleeder tank. But manual bleeding should work OK too. I'd then crack the brake lines a bit to let the air out of them and tighten as the fluid is coming out.
Then bleed the wheel cylinders and calipers.
On the rear, did you adjust the new shoes to contact the drum? Self adjusters might not be working. I usually crank the adjuster until I get some interference when putting on the drums. But I want the drums to have only a little drag when I rotate them. If you don't adjust them up, you'd have to drive the car back and forth while the car stomping on the brakes to get them to adjust up enough. Another thing to check is your parking brake cable. Is it released all the way? If not, it might make your shoes contact only part of the shoe and limit the braking force.
I don't know what you mean, brake without abs. Did you run the lines direct into the MC somehow?
I had heard about a scan tool to set the ABS too years ago. I've since found no scan tool is needed to bleed the system as long as there is nothing wrong with the ABS system. I hope this helps.

I dont have air in the mc itself until I use abs (so abs motor is cycling on heavy braking/slippery road). When I use abs, is air getting to the brakes somehow. As long I dont brake using abs (=light braking), I have no problems.

Parking brake and shoe adjustment should be good. All problems come around since abs has been used during braking.

What I dont get is where the air comes to abs module bleeder screws (those 2 around master cylinder)? I have put liters of fluid through and I just keep getting those god damn bubbles... I think if i get rid of those bubbles everything will be ok...

Been thinking about bench bleeding master cylinder but what I have understood is that the air is in the abs module itself now, not in the master cylinder?

My theory how the air enters the bleeders is similar to how moving air picks up paint in a paint sprayer. The vacuum created by the moving air sucks up paint and creates a mist of air and paint. Now the bleeder is a hole that is plugged by the tip of the bleeder which has an angle on it. When you unscrew the bleeder, the fluid rushing out of the hole draws in and mixes with the air around the bleeder and exits through the bleeder. The threads of the bleeder are not sealed so more air can get in through them. Maybe putting teflon tape over the bleeder threads could minimize or even stop the air from mixing with expelled brake fluid. I was fighting this same thing recently and I wondered why they don't put an o-ring on the bleeder to seal it.
As far as ABS and air. The ABS has a cam inside which is connected hydraulically to the brake fluid system. When the system senses a wheel is locking up, a motor turns the cam, as the cam turns, it creates space in the hydraulic system releasing pressure from that part of the system. It rotates several times per second and you feel and hear a pulsing in the pedal. Are you sure you are not mistaking this for air in the lines? The bleeders on top of the ABS are for getting air out of the system. They do work. I swapped the MC with ABS on my 96 and I did not have a problem with those retaining any air.
Try the teflon tape idea and if it works, it will assure you that there is no air in the ABS. Not unless the seal between the MC and the ABS is messed up.
But if that were the case, in the normal mode of the ABS, you'd have a soft pedal.

ifixalot wrote:My theory how the air enters the bleeders is similar to how moving air picks up paint in a paint sprayer. The vacuum created by the moving air sucks up paint and creates a mist of air and paint. Now the bleeder is a hole that is plugged by the tip of the bleeder which has an angle on it. When you unscrew the bleeder, the fluid rushing out of the hole draws in and mixes with the air around the bleeder and exits through the bleeder. The threads of the bleeder are not sealed so more air can get in through them. Maybe putting teflon tape over the bleeder threads could minimize or even stop the air from mixing with expelled brake fluid. I was fighting this same thing recently and I wondered why they don't put an o-ring on the bleeder to seal it.
As far as ABS and air. The ABS has a cam inside which is connected hydraulically to the brake fluid system. When the system senses a wheel is locking up, a motor turns the cam, as the cam turns, it creates space in the hydraulic system releasing pressure from that part of the system. It rotates several times per second and you feel and hear a pulsing in the pedal. Are you sure you are not mistaking this for air in the lines? The bleeders on top of the ABS are for getting air out of the system. They do work. I swapped the MC with ABS on my 96 and I did not have a problem with those retaining any air.
Try the teflon tape idea and if it works, it will assure you that there is no air in the ABS. Not unless the seal between the MC and the ABS is messed up.
But if that were the case, in the normal mode of the ABS, you'd have a soft pedal.

I completely agree that the bleeder screws would benefit from O-rings or even possibly Teflon tape -I've had the same thoughts over the years, but have never actually done it -somehow it works well enough as-is. The idea of comparing the paint sprayer to pulling air through the threads and expelling it with fluid is erroneous though. Paint sprayers use the Venturi effect, same as a carburetor, to accelerate air through an orifice or across the front of a fluid port which causes a pressure drop, which in turn sucks paint into the air-stream where it gets atomized (ideally) and mixed with the speeding air. With the brakes, fluid is coming out under pressure as the pedal is applied during proper bleeding, and it would try to push past the threads as well as out the port in the bleeder screw. It simply does not push through fast enough to cause a Venturi effect. Additionally, the port in the caliper (or whatever) is larger than the hole in the back-end of the bleeder screw, by design, so the fluid will have as high a pressure in that tiny pocket as anywhere else as you are bleeding it, and the fluid itself will try to push out past the threads, opposite to what you are thinking. What you have seen where it pulls air in (more typically past the seal of the bleeder hose where it sits on the bleeder nipple than through the threads themselves) is probably when gravity-bleeding or vacuum-bleeding, like with a Mity -Vac or one of those one-man bleeder cups that use fluid to siphon more air/fluid out of the brake system. Both methods cause a negative pressure in the bleeder hose and to a lesser extent the pocket at the back of the bleeder screw, and can suck air in, making it look like there is still air stuck in the brake system.

I've said it before, I will say it again, you MUST start with properly bleeding the MC FIRST, or nothing else will work right if it has air trapped in it. And using those one-man bleeders is not reliable, so unless you are bleeding the proper way with two people and only opening the bleeder screw after the pedal is being applied and making pressure in the system and is closed again before the pedal is released, you can expect to have problems. A vacuum-bleeder does work pretty well, but can still leave some air in some systems sometimes because you are never creating a positive pressure inside the braking system to push fluid out every possible point. Is your scrap-car sourced MC bad also? Maybe, in which case you can do everything right and still have a problem. The big issue with a used or worn MC is when releasing the brake pedal, the piston in the MC travels back creating a suction in the MC which pulls fluid back into the cylinder, but if the MC is worn out, dirty inside, or the seals have dried-out, it can suck air in from the back instead -and now you have introduced air in the system again, each time you apply the brakes. More than once, I have worked on a brake system that had a perfectly functioning (not 'good' though) MC, and during the bleeding process the neglected MC travels farther than it has in many years, causing the seals to rub past worn-in steps in the cylinder bore which damages the seals, or into dirt/contaminants that accumulated in the cylinder and damages or contaminates the seals, and now the MC needs to be replaced or rebuilt. Try a NEW MC, or rebuild your original, try again and BENCH BLEED it. The newer GM ABS systems do have an auto-bleeding system that can be commanded with a scan-tool, but our cars are not that new, so don't be concerned over it. As ifixalot said, the bleeders on the ABS module do work, but if the MC keeps pushing air into the module, it doesn't matter how good the bleeders are. Good luck.

"The idea of comparing the paint sprayer to pulling air through the threads and expelling it with fluid is erroneous though"

Perhaps, it is, just a theory. I have a pressure tank bleeder that I use at 10 psi. I often wonder if the fluid moves too slow
to force the air out. I imagine a bubble floating in the line drifting back up the line faster than the fluid can force it down.
I've watched a bubble in the clear line float back up as I bench bleed a MC. I figure that same could happen in the steel line.
I can only get about 1/3 turn on the bleeder to open it so there is no rush of fluid. My wrench winds up coming in contact with
the brake line. I can only get the wrench on at a certain angle because of the backing plate.
So I asked the wife to push the pedal while I opened the bleeder that 1/3-1/2 turn. The bottom of the bleeder hole has the hole in it.
The bleeder has a cone shape that seals the hole when tight. When you open the bleeder, the hole is unsealed and fluid rushes
up the cone shape and out a hole that is drilled at an angle. That hole intersects with the bleeder hole in the center of the bleeder.
I figure at 1/3 turn, the threads only allow, maybe .005" gap? I figure with somebody applying pressure on the system, that fluid
rushes out the .005" gap pretty fast, that could be causing a pressure drop as the fluid gets past the restriction of the .005"
which would then suck air from the threads.
While I bleed my 88 brakes in this manner, with the wife's help, I got air continuously from the driver side cylinder but very little from
the passenger side. I probably ran 3 oz of fluid through refilling the MC every few pumps.
Then I switched back to my tank bleeder and I had no more air coming out.
Again, it's just a theory I can't prove it.