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Old 07-26-2011, 10:24 PM   #1
npdimonte
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Question Replacing brake master w/ABS, need advice please...

I will be replacing a brake master cylinder on a '93 SL2 with ABS and have questions on how to do this. Do I need a special scan tool do this job?
I have been reading that the modulator drive gears are under spring load. Whats the best way to relieve this spring load?
Then, how do you bench bleed this master unit? there are no threads on the lower port holes. I normally connect a line from each port and route it back to the reservoir when I bench bleed the master. Do you connect the ABS unit and then bench bleed it?

Thanks for any advice, this is my first time replacing a Saturn brake master with ABS.

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Old 07-26-2011, 11:08 PM   #2
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Default Re: Replacing brake master w/ABS, need advice please...

Quote:
Originally Posted by npdimonte View Post
I will be replacing a brake master cylinder on a '93 SL2 with ABS and have questions on how to do this. Do I need a special scan tool do this job?
I have been reading that the modulator drive gears are under spring load. Whats the best way to relieve this spring load?
Then, how do you bench bleed this master unit? there are no threads on the lower port holes. I normally connect a line from each port and route it back to the reservoir when I bench bleed the master. Do you connect the ABS unit and then bench bleed it?

Thanks for any advice, this is my first time replacing a Saturn brake master with ABS.
You don't need a scan tool. I would recommend rebuilding the current master cylinder, if it's not scored. That's cheaper and easier than swapping the ABS unit on the MC (and you keep your superior OE master cylinder).

If you do replace the MC. Bench bleed the MC alone first, then bench bleed it with the ABS unit attached (first from the line outs (optional, but I'd recommend it), then from the bleeder valves, then from the line outs again).

You may also need to bleed from the caps on the MC above the proportioning valve (15/16", very shallow). These will gravity bleed--don't try to pressure bleed from them, just loosen the caps until fluid starts coming out, then tighten them again (as the highest part of that area of the system, they tend to hold a bit of air). I would do that after a brief bleeding of the ABS unit (followed by bleeding the ABS unit more thoroughly. For example. Bleed: MC, ABS via line outs, proportioning valves, ABS via bleed screws, ABS via line outs.

Finally bleed the entire system as a whole, from each wheel.

You should try to get the gears back into their original position, if possible. If you don't, it will eventually adjust back to normal by itself (after several drive cycles) but until it does, you will likely have the ABS light on, and the ABS probably won't work right.

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Old 07-27-2011, 08:46 AM   #3
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Default Re: Replacing brake master w/ABS, need advice please...

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Originally Posted by PlasticCarsRock View Post
You don't need a scan tool. I would recommend rebuilding the current master cylinder, if it's not scored. That's cheaper and easier than swapping the ABS unit on the MC (and you keep your superior OE master cylinder).
I have 205k on this Saturn and just last year the rear line rusted away and my daughter ran the fluid dry allowing the brake pedal to go all the way to the floor. I know its not leaking now, but recently my daughter claims the pedal went to the floor when she tried to stop and hit a curb. The brake pedal feels a little soft, maybe a simple rebuild will do the job, just planing for the worst though. This might be a simple fix by just rebuilding, sure sounds like I would save me allot of time.

Quote:
Originally Posted by PlasticCarsRock View Post
If you do replace the MC. Bench bleed the MC alone first, then bench bleed it with the ABS unit attached (first from the line outs (optional, but I'd recommend it), then from the bleeder valves, then from the line outs again).
When bench bleeding MC alone, do you connect lines to all four ports? or just the upper two? Where are the bleeder valves and how many are there?

Quote:
Originally Posted by PlasticCarsRock View Post
You may also need to bleed from the caps on the MC above the proportioning valve (15/16", very shallow). These will gravity bleed--don't try to pressure bleed from them, just loosen the caps until fluid starts coming out, then tighten them again (as the highest part of that area of the system, they tend to hold a bit of air). I would do that after a brief bleeding of the ABS unit (followed by bleeding the ABS unit more thoroughly. For example. Bleed: MC, ABS via line outs, proportioning valves, ABS via bleed screws, ABS via line outs.

Finally bleed the entire system as a whole, from each wheel.

You should try to get the gears back into their original position, if possible. If you don't, it will eventually adjust back to normal by itself (after several drive cycles) but until it does, you will likely have the ABS light on, and the ABS probably won't work right.
Do I need to remove the gears? if not, whats the best way to relieve the spring tension? Should I move the gears to a preferred position that will allow the system to be easily bled?

Thanks

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Old 07-27-2011, 10:28 AM   #4
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Default Re: Replacing brake master w/ABS, need advice please...

I think you need to remove or do something with the gears to remove the abs unit from the MC. I've never actually separated them, though. If you decide to just rebuild the MC, do it with the ABS unit attached, and don't worry about any of that.

First, determine if you really need to rebuild it. Try holding the pedal firmly (harder than a normal stop, but not panic stop). If the pedal stays where it is, with constant pressure, the MC probably isn't leaking. If the pedal slowly sinks to the floor, then it is.

The brakes should be pretty much the same at all times (ie. if it goes to the floor once, it will always go to the floor with constant pressure, so see if it truly goes to the floor). I'm not saying that this is what happened, but "the pedal went to the floor" is an easy excuse when you screw up (particularly if it's happened before). If you cannot replicate the pedal going to the floor, it probably didn't and won't.

Rebuilding the MC (with the ABS intact) is easy, if you decide to go that route. First get the rebuild kit (NAPA sells it, if not all local parts stores, and you can probably order it online). I think it cost $10-$20 last year. They're listed as only for non-ABS, but the non-ABS kit will work fine for you (I think the only reason it's "not for ABS" is because it doesn't include the seals that go between the MC and ABS unit (which you won't need, if you don't remove it)).

First remove the MC. If the fluid is dirty, drain it from the reservoir and ports, by propping it on an angle. If not, just drain the reservoir and leave the clean fluid in the rest of the system (MC and ABS).

Using a small flat screwdriver(s) and perhaps a small pair of pliers, remove the snap ring inside the end of the MC. This will allow you to remove the primary piston and associated seals. Next, you will need to use compressed air to blow out the secondary piston. Hold your finger over one, and blow air into the other of the forward two line-outs of the ABS unit. Start out gently, and increase in pressure until the secondary piston comes out. If you need a lot of pressure, it will come out fast, so have something in the way to stop it.

The kit will include a new secondary piston (probably) and new seals. Put the new seals on the old primary piston and secondary piston if necessary (pay attention to the direction of the old ones, when removing them). Lubricate the seals with clean brake fluid and slide everything back in, in the reverse order of how you removed them (spring?, secondary piston, spring, primary piston, snap ring).

Bleed as I instructed above. Replace in car, bleed entire system. Done.

(Sounds even harder than it is.)

Note: if the bore of the MC is scratched, you cannot rebuild it. This is very unlikely unless you somehow got something other than brake fluid in the brake system.

The two bleeder valves on the ABS unit are right on the front, you can't miss them. They only bleed the front half of the brakes, so you must finish by bleeding through the line outs. If you don't have hoses to bleed the fluid back into the MC, just bleed it into a container and dispose of it (a few pints of fluid wasted is cheaper than a bleed kit (which I don't think exists).

The caps I mentioned are on the MC, right before it attaches to the ABS unit. Just loosen them and let them gravity bleed, after a quick bleed of the rest of the system (just enough to get fluid to the line outs). Follow this up by a more thorough bleed of the rest of the system. While bleeding, hold it on an angle so the line outs are higher than the rest of the system.

Once on the car, bleed again from the line-outs, by gently loosening one line at a time, and having someone pump the pedal (or with a pressure bleeder). Then bleed at each wheel until you see clean fluid and no more bubbles.

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Old 07-27-2011, 11:14 AM   #5
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Default Re: Replacing brake master w/ABS, need advice please...

A few things I forgot:

Unless you can hold the pedal and have it sink to the floor, it's definitely worth bleeding the system on the car before you do anything else with the master cylinder. If the pedal does sink, first check for any leaks (rear wheel cylinders are most likely) before doing anything with the master cylinder.

If the pedal sinks, and you're sure there are no leaks, you should replace/rebuild the MC. Unless the bore of the MC is scored, scratched or damaged in some way (it should be perfectly smooth with no irregularities), rebuilding the MC will be just as effective as replacing it, if not more effective. Most aftermarket MCs, even rebuilt ones, are not the same as the OE one. They have the same function, but they're cast iron, instead of aluminum. This makes them cheaper, but OE used aluminum for a reason. First, it's lighter. Every little bit helps. But more importantly, unlike cast iron, aluminum doesn't rust, and oxidizes at a far slower rate. In perfect conditions (brake fluid replaced every two years, and a perfect seal at the reservoir cap), this isn't an issue--brake fluid alone does not support any kind of oxidation (although the outside of the MC will rust and look bad, if the coating is damaged by spilled brake fluid).

More importantly, if any moisture gets into the brake system, the polished iron surface will start to rust (unless you bleed your brakes every few years, you will get some moisture in there over time--brake fluid readily absorbs moisture out of the air).

Once the bore of the master cylinder starts to rust, it will need replacing (can no longer be easily rebuilt--must be bored larger and rebuilt with larger parts, which are not commercially available).

Rebuilding, while uncommon, is not difficult. The only non-household tools you'll need is a source of compressed air to remove the secondary piston. The hardest part, by far, is removing the snap ring that holds everything in (snap ring pliers cannot be used because there are no provisions for them). hold the piston in a bit (with a Philips screwdriver, for example), and pry the snap ring out with a small flat-head screwdriver. It may help to push it in, slightly, first, to get it out of the groove, but be careful not to push it in too far and scratch the bore.

I think above I said the kit comes with a new secondary piston. I meant primary piston. You can look at the new primary piston (where the first seal is, in relation to the end) to see how far you can push the ring in, without risking damaging the bore.

On rockauto, rebuild kits go from $13.30 to $17.34, with cast iron "professionally" rebuilt MCs starting at around $50. The cheapest kit (Dorman for $13.30) appears to come with both a primary and secondary piston (all seals already installed, ready to install in the MC), and a different style snap ring (one that can be removed and installed with snap ring pliers). This would be the easiest to install, and easiest to rebuild in the future, but at that price, I'm not sure how good the quality control is (still probably as good or better than what you'd get in a rebuilt one).

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Old 07-27-2011, 03:10 PM   #6
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Default Re: Replacing brake master w/ABS, need advice please...

Wow, thanks for the detailed post. I have a new MC from Rockauto, but I think I will swing by Napa after work to pick up a rebuild kit.

Thanks for the info,

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Old 07-27-2011, 03:15 PM   #7
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Default Re: Replacing brake master w/ABS, need advice please...

Just ordered the rebuild kit from Napa, non-ABS version, $21 and will pick up tomorrow.

Thanks again,

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Old 07-10-2018, 04:19 PM   #8
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Default Re: Replacing brake master w/ABS, need advice please...

Quote:
Originally Posted by PlasticCarsRock View Post
....
First, determine if you really need to rebuild it. Try holding the pedal firmly (harder than a normal stop, but not panic stop). If the pedal stays where it is, with constant pressure, the MC probably isn't leaking. If the pedal slowly sinks to the floor, then it is.
...
Should the engine be running or not?

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Old 07-10-2018, 04:33 PM   #9
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Default Re: Replacing brake master w/ABS, need advice please...

Enabling ABS is an option on my Tech 2 for my 96. Are you sure you don't have to update the PCM for older models?

-Robert

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Old 07-10-2018, 07:14 PM   #10
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Default Re: Replacing brake master w/ABS, need advice please...

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Should the engine be running or not?
It's not necessary to have large bolded lettering to ask about anything. Testing brake master cylinders requires the engine off. If you have the engine running, you cannot separate a power assist problem from master cylinder problems because the vacuum brake unit is powered up the moment the engine runs - engine vacuum provides the brake vacuum assisted unit with vacuum to apply power assisted braking to supplement leg power. Unless you know the difference, testing of brakes are always done with engine off to remove power assist from interfering with testing brake master cylinders.

Brake master cylinders rely on plain hydraulics to generate braking forces by applying the brake pedal without the engine running. To be sure power assisted braking is disabled, press and release the brake pedal several times - you'll hear a slight hssing sound as the vacuum reserve in the brake boost unit is depleted of vacuum from several pedal applications (engine must be off). As soon as one or more pedal applications occur, the pedal should begin to firm up, under normal procedures with operating master cylinders. When the pedal firms up, all vacuum brake assist is depleted and all you feel is the brake master cylinder generating hydraulic pressure. The pedal should travel about half way before stopping. You should be able to stand on the brake pedal as if your life depended on it and it should not fall to the floor. If it does, either air is in the system at the master cylinder or its worn out.

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Old 07-10-2018, 07:49 PM   #11
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Default Re: Replacing brake master w/ABS, need advice please...

I bled my ABS system with the engine not running. I had replaced all the brake lines. No big deal.

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Old 07-10-2018, 09:35 PM   #12
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Default Re: Replacing brake master w/ABS, need advice please...

^ I think you meant you were able to bleed your brakes with abs (engine off) and not bled the abs system, a separate subsystem invisible to the hydraulics when engine is off/ignition off. Bleeding ABS, to my knowledge, is not a diy or (presuming) a repair shop procedure since service manuals state GMs scantool is needed to bleed abs fluid. Either GMs Tech II scantool or high end scantool like those sold by Snap-ON or other commercial aftermarket equipment manufacturers for advanced access to vehicle electronics. Every GM service manual states engine off for bleeding/flushing brake fluid. ABS is not bled unless it's determined air entered the abs plumbing to create either an immediate abs error code or at the least, an unusual braking problem when abs is activated during an emergency stop. Flushing or bleeding the brake system is considered routine even with abs and simply requires the engine off and ignition off to ensure abs isn't turned on when ignition is on. Although it's never described this way, abs is physically and hydraulically invisible to the brake hydraulic system as long as the the engine is off and ignition is off - the same as if abs were never installed. All brake bleeding and flushing procedures remains the same whether abs is there or not.

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Old 07-10-2018, 11:21 PM   #13
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Default Re: Replacing brake master w/ABS, need advice please...

I replaced my MC on my 96 with ABS. I remember that I should not have messed with the gears on the ABS module. I think Plastic had a picture of the internal valving and set me on the correct path.

Hindsight is if you remove the ABS module do not touch it. Replace the MC and rebleed. If the self test makes the brakes side rebleed again. My 1996 thread may cover this.

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Old 07-11-2018, 07:01 AM   #14
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Default Re: Replacing brake master w/ABS, need advice please...

Also I bled my ABS MC on the car. A quickie bleed is to loosen one line at a time and use a finger to cap the port. That won't stop air or fluid from coming out but will keep air from getting back in. Not the neatest way to do it!

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