Getting ready to replace front pads and rotors on my 2000 9-3. Everything I have seen says it is quite simple. Basically push the caliper back, pull the guide pins, remove and replace rotors, and button up.
Now, a guy at work is telling me, that you shouldn't just push the calipers back on ABS, but crack the lines and bleed them as you push them back so you don't clog the orifice. Then refill fluid. I've never done this on other cars but it has been a while (about 8 years) since I have replaced pads and rotors
I have a 2001 9-3 and just finished doing front pads and rotors over the weekend. Aside from a slight problem with getting one of the clips back on (my own damn fault) everything went smoothly. I used a C-clamp to force the piston back in and have had no problems since.
Hey brother Dave - I'm not of one position or the other on this one - but I can see how it might go a some way to keeping any sludge out of the calliper circuits if you did bleed while you compressed. I think the ABS issue is irrelevant though.
You would want to be careful so that you didn't let the piston slip back and introduce a bubble or go in too fast/far and purge the bore. But - I can't see how it would hurt to carefully pressure bleed the calliper instead of reverse pressurizing the system.
You would probably want to top up the system after doing each shoe - but that wouldn't be too hard if you have a pal at hand to pump the peddle.
Yes. If your brake fluid is crudded up, then fix that, but it's not relevant to doing pads. Flush brake fluid first maybe, but these days it's not like we're driving around with DOT-3 brake fluid.
Speed won't enter into it - bubbles aren't in there, pressing fluid back up won't create them.
It's not necessary, is what I'm saying. Sure, you can bleed your brakes whenever you want, but by their nature, brakes work by fluid going up and down the brake lines in some volume, so by definition they have to allow that to happen.
Sounds like a lot of screwing around for a non-issue, to me.
Under no circumstances should you push the caliper piston back into the bore - there is definitely the potential of damage to the ABS.
The best solution is to crack open the bleeder screw (not the brake line) and slowly push the piston in with a C-clamp. Be sure to use something between the clamp and the piston, such as a piece of metal. On some vehicles, you could even use the old brake pad. Another option is to use a large socket centered inside of the caliper. Be sure to close off the bleeder prior to assembly.
After you've reassembled the brakes, press down slowly on the brake pedal, about 1/2 to 3/4 of the travel, as many times as necessary to restore proper pedal. Top off brake fluid as needed. Also, this is an excellent opportunity to replenish the brake fluid to the front wheels. Use a syringe to draw out as much old fluid from the master cylinder as possible, fill with new fluid, and then bleed each front brake until new fluis comes out. The use of a piece of clear plastic tubing attached to the bleeder screw allows you to observe the color of the fluid, as well as minimizing mess.
What I posted says it all - if one forces brake fluid through any ABS system in a direction it was not intended to flow, there is always the potential for damage. The electronically controlled solenoid valves in a hydraulic control unit are not designed for bi-directional flow. The act of forcing a piston back into its bore in the caliper can - and occasionally does - cause damage. Why even risk it if it can be avoided so easily?
This isn't a Saab issue specifically; rather it's concept that applies to cars with ABS in general. My advice is based on many years of experience and common sense. Factory repair manuals are great for providing info on how to do repairs on proprietary components, or for wiring diagrams or troubleshooting diagrams. But does one need to refer to Saab documentation to do decide if it's better to avoid damaging an expensive component by following my suggestion? I think not.
I have worked on several ABS systems from 3-4 different manufacturers and have never been aware of an ABS system that would have a problem with backing off the caliper piston.
Most if not all ABS systems are a magnetic/electronic sensor picking up off the rotor and controlling the fluid at the master cylinder point, not at the caliper point. If that is correct, then compressing the caliper piston shoud have no ill effects on the ABS system.
If brake fuild in the system wasn't meant to go bi-directional, where does the brake fluid go after we release the brakes? Thru the lines over to the other side? :)
A guide is a reference tool and logic is another good tool to use when DYI.
I've replaced rear pads/rotors using the Saab-authorized (Bentley) repair manual, and it makes no mention of damaging the ABS. It recommends bleeding the rear brakes using the ABS pump (which works really well btw), but it instructs you to back the caliper off using the 4mm (?) hex screw (which should, I think, be the same thing as pushing the piston back). I realize the front brakes are a little different, but I've read the procedure and do not remember reading anything about potentially damaging the ABS. (I don't have the book on hand to check at the moment.)
When the car is running and the ABS system is energized, the electro-magnetic solenoids are energized and the control valves are open. Therefore, the minute amount of pressurized fluid required to move the caliper a few thousandths of an inch can flow. And when the brakes are released, the return action of the caliper piston seal and retraction of the master cylinder piston does indeed cause that same minute amount of fluid to return to the master cylinder. But remember, the valves are OPEN. This is by design.
When the ABS system is not energized, the valves are closed. Why would anyone want to risk damaging the seat of these valves by forcing brake fluid against them? If the seat is damaged, the anti-brake function is compromised, as a leaking seat would not permit the hydraulic pressure to the "locked up" wheel to be sealed off and diverted.
I'm afraid your idea of how ABS works isn't correct. Even with ABS the system is *always* vented to atmosphere at the fluid reservoir with the pedal is in it's resting position. There is always a free path for fluid to return from the wheel cylinders to the reservoir without needing to "force" any valves open.
Just using the brakes normally, with or without activating ABS, creates vastly more pressure in the system than pushing the wheel pistons back by hand ever can.
I had not previously weighed in on this thread, mostly because I am not particularly well versed in ABS pump design. OTOH it seems that you are, Doug.
I must confess that I have used the C-clamp method on a number of cars with ABS (including late model SAABs) and I've at least "gotten away with it", so perhaps these valves you mention are more hardy than you might imagine. At least anecdotal evidence seems to support that since there are a bunch of us testifying we have done so with no ill effect. In fact, I've yet to ever hear anyone say they botched their ABS by pushing back the calipers.
In any case, based on your above description, could you not then simply turn the car ignition to on to activate the ABS valve, which would put it in the open position while you then push the caliper piston back? That would seem to satisfy your desire to not damage the valve(s) and at the same time leave the system sealed so as not to introduce air into the caliper circuit. Perhaps the best of both worlds?
I believe that the "dirt in the system" argument is specious. If there is dirt in the system it will migrate throughout regardless of whether we push fluid back or not. Certainly, one should plan on flushing the brake fluid at regular intervals anyway, and this would mean there is no dirt.
I won't be changing my process until/unless I see something from the people who make the cars. Otherwise it's exactly the same thing as calling your ISP's tech support people, saying "your newsserver is down", and being told to reboot your home PC. Has nothing to do with it, but "we always say that, just in case". Doesn't follow.
I'm now driving an Audi (just couldn't bring myself to buying a GM Saab after many pleasurable years with 99's and 900's) and the recommendation from their workshops is the same - open the bleed screw as the piston is returned, for either front or back brakes.
Their argument is that if there are contaminants that have collected inside the pistons (quite likely after a couple years' use), they would travel up to the ABS and contaminate the valves there.
I haven't heard the info you'd collected about damage to the solenoid valves due to flow; I kind of doubt that fluid travelling in different directions would harm anything. *Clean* fluid, that is ...
We may as well leave it up to the OP what the best course of action is, when pushing the pistons back. Some here find no use for bleeding off the fluid; fine, let them do what they want. I, for one, bleed the fluid because I'm convinced that the fluid upstream of the ABS (i.e. closest to the reservoir) is cleaner than that in my brake pistons.
Another possibility may be that it might not be necessary to bleed, as long as the car owner's been a good little boy/girl and had the old fluid flushed every 2 years.
...that specific question, so I will file your advice as "some guy on the Internet said", unless I get some sort of, you know, actual information.
In other words, thanks for your guess, but until you can back it up with anything other than more of your guesses, following your guess would be useless at best.
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