weak brakes with ABS

[ganda1f]

To cut a long story short, can the pressure modulator in ABS brakes cause a pressure drop in brake lines when ABS is disabled, e.g. by pulling the fuse? Given that the modulator itself is not defective. I don't suppose I can diagnose it myself, the unit being sealed precision assembly and all.

Comparing two similar cars, one with ABS and the other without. The brake pedal has to be pushed really hard and quite far before the brakes start to lock up on the car with ABS. ABS being disabled at the time by removing the fuse. I have ruled out worn pads, malfunctioning calipers, air in the system and differences in tires. The brake pedal feels firm right from the start once the vacuum in brake booster is released.

Any tips for further diagnosis are greatly appreciated.

Here is the long story:
Brakes on a Toyota Carina '97 2.0l 3SFE

[new echo owner]

Why do you want to lockup the brakes?

The idea behind ABS is not to allow the wheels/tires to lockup, preventing the tires from loosing traction. Unless the ABS computer detect an inpending lockup from one of the wheels it won't engage/activate. Usually, if there is a problem with the ABS system, the warming light will come up, no warming light no problem in most cases.

N.E.O.

[ganda1f]

I don't want to lock up the brakes, it is just that the car does not feel like it is braking hard even though the pedal is pushed hard and quite far. The ABS (when enabled) pretty much does not engage either, no/very little pulsation in the pedal. So I disabled ABS to see if it was somehow interfering silently. Then I still had to push the pedal quite far before the wheels would lock up. On a non-ABS car when slamming the brakes one gets thrown forward, but not in this case. I am just trying to figure out if this behaviour is there by design or because of some malfunction.

[new echo owner]

How many miles is on the car? Factory brake pads?

N.E.O.

[ganda1f]

No, new pads. Changed them myself some 2000 miles ago, no difference. Brake shoes in the rear are fine as well. The same kind of pads is used on the other car without ABS and they are grabbing strong. Sliding pins in the calipers are fine. Thought about pistons in the calipers, but if these were stuck, the brakes would drag. And they don't. The car has 90k miles.

I read about the low pressure behind the ABS modulator on one of the TN truck forums. But that guy had way more severe problems, could barely keep the truck from rolling on steep hills. So I thought that perhaps the difference I am noticing is related.

[new echo owner]

Are they factory pads? ABS and non-ABS vehicles don't always use the same pads, some thing to do with the friction coeficient or some thing like that.

N.E.O.

[ganda1f]

Do you mean OEM? No, not Toyota pads. Previous pads were Toyota, but I guess they could potentially install the wrong kind. Considering that there is no difference in behaviour after the swap.
The annoying part is that I am actually not even sure if this is a problem. Maybe the brakes are just set up in this way and that's it. I just think that the difference between ABS and non-ABS is weird.

[new echo owner]

Check with a dealer to see if they list different pads on ABS and non-ABS vehicles.

Compare it with another Carina with the same set up, if you are not sure whether that is the way it is designed.

N.E.O.

[ganda1f]

I think I got this issue sorted out now.
I found a thing called brake pedal travel reserve, which is the distance measured from the upper face of the brake pedal to the floor (carpet removed) when the brakes are fully depressed. The distance was just within specs (approx 65mm). But I decided to take apart the rear brakes even though they looked to be fine. Cleaned them, lubed the shoes-to-backing-plate contact points. And I could see an improvement, the reserve distance was now more like 75mm. The brakes do feel stronger after this. At least I have no reason to doubt them anymore. Guess the shoes could not move as they should.

[new echo owner]

Not familiar with the setup of the Carina, but rear brake shoes adjustment does affect brake pedal height.

Glad to hear that you have sorted it out.

N.E.O.

[jonesufg]

thanks you for your post, you help is appreciated