Intermittent Brake Problem

[The Critic]

My parents and I drive a '96 Saturn SL2 with ABS and 4-wheel Disc Brakes.

There is an intermittent problem with excessive brake pedal travel. Most of the time, the brake pedal is fine. It isn't super firm, but it's acceptable. However, sometimes the brake pedal has excessive travel. When this occurs, the brakes will not engage until the pedal is nearly to the floor!

The intermittent nature of this problem is what stumps me. I'd say 80% of the time the brakes are working perfectly fine. It's the other 20% of the time that's concerning. Even when the brake pedal does have excessive travel, the brakes work fine-- it's more of annoyance than anything.

I've tried two tests to check the condition of the master cylinder.

1. Pump brake pedal several times with the engine off, then push and hold down pedal and check for a drop. It passes this test.

2. Pump brake pedal several times with engine off, then push down brake pedal very slowly to see if it goes to the floor. I tried this, and the pedal goes down lower than in test #1, but the pedal doesn't go completely to the floor.

Here's some brake service history that may also be relevant:

1. Flush/bleed brake fluid-- Did this in June 2007 and in Sept 2008.

In June 2007, I used a Vacula to vacuum bleed the system. It's possible that I may have introduced air to the system, I don't know.

When I flushed the brake fluid again in Sept. 2008, I used a Motive Pressure Bleeder and removed a few air bubbles. After the Sept. 2008 flush, the pedal feel improved significantly and the pedal feel was very acceptable, so I don't think air is the issue. Also, the 1 year old brake fluid that came out was very dark-- almost like coffee colored. The bottom of the used brake fluid container also had some bits of black, gritty rubber seal like material. I wonder if it's from the master cylinder?

2. Replaced rear pads and rotors. My original pads lasted until 90k and they still had ~10k left. However, my second set of pads only last 28k before they wore out. The wear was perfectly even as well...no signs of uneven wear aside from the normal slight difference between the inner and outer pad.

So, I was wondering if this is a sign that the proportioning valve in my master cylinder has failed? (The proportioning valve is built into the master cylinder on this car)

And as far as I know, all of the caliper slides are working properly. The right rear caliper piston was a bit on the difficult side to retract at the latest brake job, but I think it may have been caused by me using the wrong tool on the "cube."

Thanks in advance for your assistance!

 

[Shannow]

Intermittent and air bubbles don't sound consistent to me.

 

[Tosh]

Sounds like an internal leak past the primary piston seal.
Howstuffworks page

 

[Gary Allan]

The rear pad wear is odd.

Normally, if it was a proportioning valve issue, I expect you to be reporting the rear locking up, but with the ABS, this should not occur.

 

[oilyriser]

Isn't there a check valve between the reservoir and the master cylinder that allows fluid to flow down to replenish the system but not back up? If that sticks open, fluid would be pumped back into the reservoir instead of to the wheel cylinders.

 

[eljefino]

^ usually that check valve is a slit that's only uncovered when the pedal is at rest.

I would (sigh) bring it to a GM dealer or shop with Tech II and have them rebleed while doing the "actuator SOP". If they don't know what that is... RUN!

 

[mechtech2]

Probably a master cylinder that leaks internally.
Sometimes it is OK, and sometimes it passes fluid instead of compressing it.

It it was air, you would feel it all the time.

Another possibility is moisture in the fluid that is boiling, but it would have to be really hot.

 

[daman]

faulty master cylinder...

 

[labman]

Could be some sort of a hardware problem. If a piston is pulling back for some reason, it will cause low pedal.

I may be parting company from Oreily's. Last year I bought a bunch of stuff there including kits to rebuild the calipers on my Cavalier and a clutch for my truck. The pistons pulled back with the Orielys Brake Best kits. The problem went away after I put in new kits from NAPA. Then the week before last, the throwout bearing in the clutch want bad in less than a year. I was lucky to get the truck home and in the garage. Changing throwout bearings is no fun. The new one came from NAPA as well may many of my parts for now on.

 

[FastAuto]

Your Saturn utilizes a Delphi VI ABS (some call it Delco) system and is a diagonally split hydraulic braking system. There are two proportioning valves in the master cylinder, one for each rear wheel. The parking brake is integral in each rear caliper. The Delphi VI ABS utilizes two isolation solenoids on the top of the modulator for the front ABS circuits only. The ABS system functions with four pistons that move up and down to release and reapply fluid to the calipers. The rear circuit utilizes check balls to isolate the rear circuit during ABS function.
The master cylinder (other than the prop valves) is a standard dual piston master cylinder. There is no quick-take-up valve in this master cylinder on the primary circuit.

Now two questions for you:
1. How did you retract the rear pistons when you replaced the rear brake pads?
2. Does the ABS light ever come on? If so when and how often.

If air was in the system the pedal would be low and spongy all the time.

Prop Valves would not cause a low brake pedal. If they or one was stuck open all the time (not functioning like it should) the ABS would activate when applying a medium brake application. The ABS would be seeing one or both rear wheels locking up on normal brake apply. This would activate ABS function (hold, release, reapply) to both rear wheels at the same time (normal for this system). What would not be normal is ABS activation on the system during a normal braking application.

Like the other post you can not rule out a master cylinder. Sometimes a slight cut in a primary cup or the secondary cup on the secondary piston may cause the issue. The small slice seals sometimes and at other times passes fluid.

 

[The Critic]

FastAuto, thanks for the great info!

1. I retracted the rear pistons with the "cube." That's what most people use on the Saturn fans forum and they have not had any issues.

2. I've never had the ABS light come on, except for the self-test upon vehicle start-up. The ABS's diagnostic self-check function also works properly at 7 mph.

 

[FastAuto]

I would check the rear calipers to make sure the are adjusting properly and taking out the wear clearance. (Parking Brake Mechanism) The inside back of the piston utilizes a sprung screw and nut type mechanism to keep the piston adjusted as the brakes wear. If this is sticking because of sludge in the caliper it could cause an intermitten low pedal until it frees up and readjust itself. If the rear calipers are working good then your next guess would be the MC.
But I would fully chech the system before spending money. This is just a few suggestions.