No brakes when car is running

[Analog Kid]

Hi Everyone,

Normally I would not post a question about a car other than a Mopar on this site however I feel that the collective knowledge here is far better than what is available through other online resources for this type of vehicle. You guys know a lot about auto mechanics so I'm going to give it a shot.

The car is a 2001 Pontiac Aztek with a 3.4 LA engine. After sitting for 3 years I am trying to get it back on the road to give to my son. I recently had to replace all the brake lines due to a leak and also replaced the front brake hardware, hubs and CV axles. When the car is off - the brake pedal is firm. When I start the car - the pedal goes to the floor. No ABS codes are present.

Here is what I have done so far to try and fix or diagnose this:

Manually bled the brakes - did this with a vacuum bleeder and the two person method.

I bought a scan tool that allows me to bleed the ABS module so I have done that about 3 times.

Adjusted the rear brakes.

Undid the master cylinder lines and capped them off. Pressed on the pedal and it is hard as a rock. I assume that would tell me that the master cylinder is good - ?

Now...I am suspecting that maybe the brake booster is bad however I would think that if it was then the pedal would be hard when the car is running - not soft. I also unplugged the vacuum line and the pedal got hard so I think the booster is OK. Has anyone ever experienced a bad booster where the pedal went to the floor when the car was started?

My other thought is that maybe the ABS module is bad but I would think I would get a code on that. Also if the ABS pressure valve unit itself was bad then wouldn't that give a code?

Any thoughts here? Appreciate the help....

Thanks,

Dave

 

[3175375]

Hi Everyone,

Normally I would not post a question about a car other than a Mopar on this site however I feel that the collective knowledge here is far better than what is available through other online resources for this type of vehicle. You guys know a lot about auto mechanics so I'm going to give it a shot.

The car is a 2001 Pontiac Aztek with a 3.4 LA engine. After sitting for 3 years I am trying to get it back on the road to give to my son. I recently had to replace all the brake lines due to a leak and also replaced the front brake hardware, hubs and CV axles. When the car is off - the brake pedal is firm. When I start the car - the pedal goes to the floor. No ABS codes are present.

Here is what I have done so far to try and fix or diagnose this:

Manually bled the brakes - did this with a vacuum bleeder and the two person method.

I bought a scan tool that allows me to bleed the ABS module so I have done that about 3 times.

Adjusted the rear brakes.

Undid the master cylinder lines and capped them off. Pressed on the pedal and it is hard as a rock. I assume that would tell me that the master cylinder is good - ?

Now...I am suspecting that maybe the brake booster is bad however I would think that if it was then the pedal would be hard when the car is running - not soft. I also unplugged the vacuum line and the pedal got hard so I think the booster is OK. Has anyone ever experienced a bad booster where the pedal went to the floor when the car was started?

My other thought is that maybe the ABS module is bad but I would think I would get a code on that. Also if the ABS pressure valve unit itself was bad then wouldn't that give a code?

Any thoughts here? Appreciate the help....

Thanks,

Dave

Might be the booster or it doesn’t have vacuum.

I also found this response to a question similar to yours:

2003 Pontiac Aztek Brakes: I Changed Booster and Master Cylinder, ...

 

[Davea Lux]

A couple of questions:
1.) Does you ABS pump start running when the pedal goes to the floor?
2.) Is the car equipped with front and rear discs, or just front discs?

Given that this car sat for three years, a lot of things could be wrong. Unequal pressures generated by a defective master cylinder on the split brake system can cause the ABS system to deploy to try and compensate for what it is reading as unequal braking. A bad ABS sensor on one or more wheels could also be tripping the ABS system. GM ABS systems would often cause the brake pedal to drop when the ABS pump was running. A bad booster can also trip the ABS if brake pressures are sensed as inadequate. If the rear axle has drum brakes be sure that they are properly adjusted. A leak from a caliper or wheel cylinder can also trick the ABS into deploying. On a vehicle with an extended lack of use should really have the wheel cylinders,calipers and master cylinder rebuilt or replaced as they are all likely corroded, especially if the brake lines were also corroded. Check these items and report back.

Dave

 

[Big_John]

When the car is off - the brake pedal is firm

Undid the master cylinder lines and capped them off. Pressed on the pedal and it is hard as a rock.

Should be as hard as a rock with the engine off. Just about the same as plugging the MC.

Try plugging one side of the MC at a time. That will tell if it's front or back.

 

[bigmoparjeff]

It's not the booster. The only failure mode on a booster is lack of boost and hard pedal.

Sounds like you have air trapped in the system somewhere. The link from 3175375 says it could take three quarts of fluid to get it bled out. That's a lot of fluid! I've never encountered a vehicle that was that problematic to bleed.

 

[Davea Lux]

It's not the booster. The only failure mode on a booster is lack of boost and hard pedal.

Sounds like you have air trapped in the system somewhere. The link from 3175375 says it could take three quarts of fluid to get it bled out. That's a lot of fluid! I've never encountered a vehicle that was that problematic to bleed.

I am thinking that there is a bad master cylinder or an air leak some place in the system.

Dave

 

[Analog Kid]

1.) Does you ABS pump start running when the pedal goes to the floor?
2.) Is the car equipped with front and rear discs, or just front discs?

Answer for #1 - No the ABS does not run when the pedal hits the floor.
Answer for #2 - It has front discs and rear drums.

I would think if there was an issue with the ABS then I would be getting a code. When I first got the car started again I had an ABS code of C0236 which is a missing rear wheel sensor circuit. I took a meter to the rear wheel sensors and got an AC voltage when I spun the wheels so they checked out OK. I then realized that the speed signal was not getting to the EBCM so I looked around and found some wires that has been chewed up on the lower engine harness. I repaired them and the code went away. I think it was the crankshaft position sensor wire since I also had that code (P0336) and it went away too.

Doing the ABS bleed is tricky because - yes a lot of fluid is getting pushed out and it can cause the MC level to get low. I've had the low fluid light come on a few times and have had to quickly refill. When doing the procedure the scan tool tells you to open a specific bleeder valve and continually pump the brakes. After about 2 minutes the ABS kicks on and you keep pumping for another 2 minutes. I am wondering if only the solenoids for that wheel are being activated or if the tool activates all the wheel solenoids. If only certain wheel solenoids are being activated then hopefully they are the correct ones for the bleeder that is open. Also the procedure likes to kill the battery....which is frustrating because the battery is only a year old.

My next step will be to do as John said and plug one side of the MC at a time and see what happens. Then I will do the ABS bleed a few more times....guess I better get more brake fluid.

Thanks,

Dave

 

[Snotty]

Don't you understand? It's an Aztek; the car is so ugly it's attempting suicide!!

 

[cbarge]

1. Key On Engine Off (KOEO), cycle the Antilock Brake System (ABS) unit with a scan tool.

2. Bleed brakes at the wheels, 2 man bleed method.

3. Tap on the Brake Pressure Modulator Valve (BPMV) with a mallet, while attempting to bleed brakes.

4. Check for good brake fluid flow from each wheel's bleeder when bleeding.

5. If low fluid flow is noted from a diagonal, such as Left Front (LF) and Right Rear (RR), then suspect a faulty master cylinder.

 

[cbarge]

1. With the pressure bleeding tool at 206 to 241 kPa (30 to 35 psi), and all bleeder screws in closed position, select Automated Bleed Procedure on the scan tool and follow the instructions.
2. The first part of the automated bleed procedure will cycle the pump and front release valves for one minute. After the cycling has stopped the scan tool will enter a “cool down” mode and display a 3 minute timer. The auto bleed will not continue until this timer expired, and cannot be overridden.
3. During the next step, the scan tool will request the technician to open one of the bleeder screws. The scan tool will then cycle the respective release valve and pump motor for 1 minute.
4. The scan tool will repeat step 3 for the remaining bleeder screws.
5. With the bleeder tool still attached to the vehicle and maintaining 241 kPa (35 psi), the scan tool will instruct the technician to independently open each bleeder screw for approximately 20 seconds. This should allow any remaining air to be purged from the brake lines.
6. When the automated bleed procedure is completed the scan tool will display the appropriate message.
7. Install all four tires, if necessary refer to Tire and Wheel Removal and Installation in Tires and Wheels.
8. Remove pressure from the pressure bleeding tool and then disconnect the tool from the vehicle.
9. Depress the brake pedal to gage pedal height and feel. Repeat steps 1 through 8 until the pedal is acceptable.
10. Remove the scan tool from the DLC connector.
11. Install .
12. Lower the vehicle.
13. Inspect the brake fluid level in master cylinder.
14. Road test the vehicle while making sure the brake pedal remains high and firm.

 

[Davea Lux]

1. With the pressure bleeding tool at 206 to 241 kPa (30 to 35 psi), and all bleeder screws in closed position, select Automated Bleed Procedure on the scan tool and follow the instructions.
2. The first part of the automated bleed procedure will cycle the pump and front release valves for one minute. After the cycling has stopped the scan tool will enter a “cool down” mode and display a 3 minute timer. The auto bleed will not continue until this timer expired, and cannot be overridden.
3. During the next step, the scan tool will request the technician to open one of the bleeder screws. The scan tool will then cycle the respective release valve and pump motor for 1 minute.
4. The scan tool will repeat step 3 for the remaining bleeder screws.
5. With the bleeder tool still attached to the vehicle and maintaining 241 kPa (35 psi), the scan tool will instruct the technician to independently open each bleeder screw for approximately 20 seconds. This should allow any remaining air to be purged from the brake lines.
6. When the automated bleed procedure is completed the scan tool will display the appropriate message.
7. Install all four tires, if necessary refer to Tire and Wheel Removal and Installation in Tires and Wheels.
8. Remove pressure from the pressure bleeding tool and then disconnect the tool from the vehicle.
9. Depress the brake pedal to gage pedal height and feel. Repeat steps 1 through 8 until the pedal is acceptable.
10. Remove the scan tool from the DLC connector.
11. Install .
12. Lower the vehicle.
13. Inspect the brake fluid level in master cylinder.
14. Road test the vehicle while making sure the brake pedal remains high and firm.

Almost sounds like it would just be simpler to bypass the ABS. Units of this vintage were not noted for a high degree of reliability to start with.

Dave

 

[cbarge]

Almost sounds like it would just be simpler to bypass the ABS.

At my work,we are not allowed to bypass. But we suggest to clients to go home and unplug the ABS motor. Yes the ABS light wil be on but at least the service brakes will function normally.
We had installed used ABS modules with motors just to get the service brakes functioning even though the module was no good.

 

[3175375]

...Yes the ABS light wil be on but...

A small piece of black electrical tape strategically placed will be the frosting on the cake.

 

[Analog Kid]

Don't you understand? It's an Aztek; the car is so ugly it's attempting suicide!!

Yep...and it's trying to take me with it.

 

[Analog Kid]

With the pressure bleeding tool at 206 to 241 kPa (30 to 35 psi), and all bleeder screws in closed position, select Automated Bleed Procedure on the scan tool and follow the instructions.

This is directly from the service manual. Great piece of literature.....6000 pages and no table of contents. I have the electronic copy.

I am assuming this was written for a GM Tech II scanner. I don't have that and won't be getting one. It costs more than what the car is worth. Instead I'm using an Autel Maxi-Check Pro ($216 at Amazon). It does the ABS bleed however the procedure is a bit different. It tells me to pump the brakes after opening each bleeder valve. The sequence is LF,RF,LR,RR. Since the service manual doesn't say anything about pumping the brakes I am going to just press on the pedal the next time I do it. Then I will try a manual bleed. I will also try banging the pump with a mallet. Sounds like fun.