Started getting a very soft brake pedal - so our customer brought her car to us and we bleed the whole system and found the RF and LR have no fluid and no pressure and we now have a very bad brake pedal - We also took the ends off at the master cylinder and fluid is squirting out fine there so the master cylinder is good. All that is left is the ABS Modulator - and he did scan bleed the modulator and still nothing - any suggestions or is the ABS modulator no good - thanks christine

edit - No codes - No work done recently -

 

No clue how to help other than Google the issues, possibly you will find a post related to a Malibu, or G6. The cars share many of the same parts. There are some pretty smart people on here. Good luck

 

I am NOT a brake expert by any means. Following any advice is at your own risk:

What is the mileage and how worn are the rear brakes, and how is the rear brake wear compared to the front?

A cross F/R brake issue screams master cylinder to me.

Have you flushed and refilled the entire system? Any debris/rust found?
When you refill and try to bleed (standard method at bleeder valves), is that when you find again there is no fluid in the RF/LR lines?

1) Try reverse bleeding the entire system

Reverse Bleeder

if you can't "backfill" the lines w fluid and see the air bubbling up through the master cylinder, either the lines are blocked or the master cylinder is bad

2) Just because the master cylinder shoots fluid in an unpressurized situation does not guarantee that it operates properly (valve wise) when the system is placed in a pressurized environment /pressurized system

If there is no fluid in those lines nor the related portion of the ABS setup (if the modulator was bad, where would the fluid go anyway?) , there is a bad seal or valve on the master cylinder such that no pressure is being generated in that potion of the brake system

You may want to investigate if that master cylinder is shared w the clutch as it was on the Saturn Ions. I have no idea but it is a potential source of a pressure leak

 

Thank you for the responses - we had it towed to the dealer and they replaced the master cylinder - but our customer still says the pedal doesn't feel right - so time will tell

 

Don't ask me to verify this or where I read it. I may not have even read it, I might have dreamed, but I seem to recall a similar story one time where someone was losing brake fluid and it end up that the brake fluid was getting into the booster assembly. They replaced the MC and booster and everything went back to normal. Hope that just replacing the MC works.

 

dang it

You weren't dreaming.

I forgot all about that. Apologies for not having mentioned it.
Although I would think that if that were the case it would affect both halves of the MC to the same extent.

 

Thanks but she wasn't low on fluid - just couldn't get any fluid to flow to the rf and lr and we bleed the master cylinder and fluid was flowing to the abs modulator so that is why we didn't think it was the master cylinder. So we will see.

 

If the fluid goes no further than past the abs modulator in a non pressurized system, my guess is a "restriction" or "pressure leak "could be involved in he the form of
--- a master cyl seal where the fluid leaves he MC for those two wheels
--- a plug of debris where the fluid leaves the master for those two wheels,
--- a blockage or malfunctioning of the proportioning valve, though from my understanding this controls frion-to-back brake pressure ratio and is highly unlikely
---------------
ABS components
--- ABS modulator for those two wheels COULD be defective in such a way that under a pressurized load (bleeding counts even though the reservoir lid is off, a small but positive pressure must be generated to have flow through the lines), the modulator is sticking in the CLOSED position and blocking all flow to the two lines in question.

This would indict EITHER the solenoid which controls the modulator position (stuck Closed) OR the modulator itself being physically defective.

Is the ABS light or Brake light on the dash lit when running? The ABS system has its own control module, but you have to hook up a scanner that can interface with it to extract any codes present and then interpret them.

Remember that once the ABS light illuminates, the entire ABS system is disabled, so physical troubleshooting will not be of much value since the system is disabled. However, NORMAL HYDRAULIC BRAKING FUNCTION IS STILL PRESENT, which allows you to do basic functionality and leak tests for the non-ABS portion of the system.

No ABS light (as indicated in 1st post) is baffling unless the ABS control modules do not sense flow from each half of the MC, It is my understanding that that piston travel for both halves of the MC IS monitored and should throw an ABS code if an issue.
_______________
1) Is ABS light on NOW when car is running?
2) Have you taken it on a test drive and purposely triggered the ABS in order to see if it will throw codes?
3) I know you said the dealer replaced he MC but pedal feel is crap. Is there now fluid at those two calipers? If so, MC was problem -- prob just needs proper bleed.

You can be nice to your customer and have them bring the car in for a free "postanalysis" with a free brake bleed and find out for yourself if the MC replacement actually fixed the problem.

We'd all learn greatly from that (hint), and given the amount of time I've put into my posts ( this post in particular), I'd really would like to know the answer...(quite intrigued)

I will obviously continue to help if necessary...just would like to eventually get a conclusive answer to what the problem was and what wasthe solution.

Thanks

Derf

PS: Everyone please review this post and point out any mistakes/flaws in logic. I am NOT an ABS expert (though I may approach one before this is over )

The LAST thing I want to do is lead you down the wrong path

 

There was no abs light on and no ABS codes - we could control the ABS modulator with our scan tool but it would not bleed or bring the ABS pistons to there home positions we are assuming since it is a sealed system. I will have her make an appoint so that we can do a post inspection like you said to see if that really fixed her problem and get a copy of the invoice of what was actually done - She said it cost her 700.00 and from what I figured it should only have costed 4 - 500.00.