I have heard a few times that you cannot push fluid backwards through an ABS system, or you damage it. Can anyone tell me what you damage and why? Doesn't fluid move both ways normally?
I have never had to deal with it before, but the time is coming when I will have to work on one so I thought I would ask.

I am not positive but I think it can damage the ABS hydraulic unit by allowing flowback.

also take care some ABS systems store fluid in an accumulator which is under very high pressure.  Make sure you check to see how to depressurize yours first.

Thanks for the advice on the accumulator. I just read about it. I am not sure if a Pathfinder has one, but I will look for it.

The real problem is not the fluid going backwards througth the system, but the contaminants that accumulate behind the pistons in the calipers and wheel cylinders. These contaminants are from burned brake fluid(Brake fluid can at times vaporize on the back of the pistons due to heat) , Rust(water contamination) and rubber contaminants(the seals and hoses gradually deteriorate). When ever you force the pistons back in to the calipers you should let them expel these contaminants from the system instead of back up towards the master cylinder. This applies to all brake systems. On ABS systems the contaminants can be large of of significant quantity to freeze up the various check valves throughout the system. The best way I have found to bleed the system is a one man bleeder kit. This is essentially a hose that you pop on to the bleeder and then drop it into a jar of brake fluid. The contaminants are forced out, fall to the bottom of the jar, then brake fluid is drawn back in. This all but eliminates air bubbles and gets a lot of the contaminants out. Do not use this jar of brake fluid to fill at the master. Keep is sealed to prevent moisture contamination.
All that being said, there are specific bleeding procedures for all vehicles. I suggest getting a vehicle spcific manual for your that shows this. Another source that I like is www.alldatadiy.com(check you technical service bulletins, much fun for some). On some vehicles this can get involved. Some later model vehicles require a diagnostic computer to run the bleed sequence. This is mainly on domestics, usually imports are simpler.

genius91

The reversal of brake fluid in a vehicles braking system is a neccesary function in using the brakes.

1. When you apply the brakes, the cylinder at the wheel gets more fluid pushed into it.

2. When you let-up on the brake pedal, fluid comes (not under high-pressure, mind you) back into the reservoir. It has gone backwards through the lines.

Most ABS systems use "bleed-off" to control braking at each wheel. YOU supply the pressure, but the ABS controls which wheel(s) get that pressure by locking the line and bleeding (slowly) off the pressure in the selected line(s)...All done in a millisecond =)

I know of no ABS systems that can be damaged by back-flushing.

I asked a couple of suppliers of ours for links to free info.

www.cardone.com
Sign up for the Pro-Tech club, it's free.

www.federalmogul.com/fmeconnect/training/index.html
It's free also, but not as good as Cardone's site.

From DA :I know of no ABS systems that can be damaged by back-flushing.

Trust me, we sell a lot of needless replacement parts to folks who thought that.

genius91

Andrewh

I'm a tech at a Nissan dealer. Email me or post here about what you're trying to do and I can give specific info about your Pathfinder. If your just replacing disc brake pads and pushing the caliper pistons back into the calipers you are ok.