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Old 04-22-2018, 10:25 AM   #1
raymondjiii
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1994 SC2
Default Why does Haynes say do not bleed rear calipers without a scan tool

I've had this car since 1994 and I've always bled the rear calipers. I have ABS but Haynes and other manuals say do not bleed the rear calipers since a mfg scan tool is needed.

That's not to say it would not be a good thing to so - I'm just curious what exactly is the need?

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Old 04-22-2018, 10:27 AM   #2
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Because the ABS module for that generation of GM cars is 100% junk.
Some people can get lucky and get the rear pistons (in the module, not the caliper) in the right spot so you can bleed the rear calipers but the only surefire way to do it is with something with ABS interactive ability like the Tech II.

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Old 04-22-2018, 12:03 PM   #3
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

raymondjiii, you bled your rear calipers and haven't crashed? Must be luck or you're doing it right despite what Haynes states. In everyday driving whether abs is used or not, the abs module usually stays in home position with all solenoids off. This means the abs system remains invisible to the hydraulic brake system. Braking is normal. When bleeding brakes, it's presumed the engine is off and brake flushing/bleeding procedures remains the same whether a car has abs or not. Since you never crashed or noticed anything unusual after bleeding rear brakes, you can be assured that you haven't done anything wrong.

Brakes work great? Ignore that part of the Haynes manual.

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Old 04-22-2018, 02:48 PM   #4
raymondjiii
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

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Originally Posted by fdryer View Post
raymondjiii, you bled your rear calipers and haven't crashed? Must be luck or you're doing it right despite what Haynes states. In everyday driving whether abs is used or not, the abs module usually stays in home position with all solenoids off. This means the abs system remains invisible to the hydraulic brake system. Braking is normal. When bleeding brakes, it's presumed the engine is off and brake flushing/bleeding procedures remains the same whether a car has abs or not. Since you never crashed or noticed anything unusual after bleeding rear brakes, you can be assured that you haven't done anything wrong.

Brakes work great? Ignore that part of the Haynes manual.
Nice. I had a good chuckle out of that.

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Old 04-22-2018, 08:12 PM   #5
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

I have a tech two but have never used it to bleed brakes. We bleed the fronts and then the rears, even when just doing rears. I wonder if using the computer would help.

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Old 04-22-2018, 08:29 PM   #6
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

The reason for that requirement relates to the older ABS system that requires homing the pistons if you happen to make a hard stop coming into the service bay or garage. If you make a gentle stop after driving at over ~10mph it is not necessary. If you have the later ABS or no ABS it is just another useless Haynes piece of bad information.

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Old 04-22-2018, 09:27 PM   #7
raymondjiii
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Quote:
Originally Posted by Tom Walsh View Post
I have a tech two but have never used it to bleed brakes. We bleed the fronts and then the rears, even when just doing rears. I wonder if using the computer would help.
Not that Iíve ever followed it, I never have, but I thought there was an order like right front, left rear, left front, right rear...some type of cross pattern. As I said I never did that but I think I usually do the rear calipers first. I canít remember my rationale.

My master cylinder is pretty much empty though with the new brake lines in but no bubble flange fittings yet for the caliper hoses. It should be interesting but I hope not.

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Old 04-22-2018, 09:29 PM   #8
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Quote:
Originally Posted by OldNuc View Post
The reason for that requirement relates to the older ABS system that requires homing the pistons if you happen to make a hard stop coming into the service bay or garage. If you make a gentle stop after driving at over ~10mph it is not necessary. If you have the later ABS or no ABS it is just another useless Haynes piece of bad information.
Interesting Nuc. With a 94 I assume Iíve got the older.

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Old 04-22-2018, 09:54 PM   #9
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

The scan tool is recommended due to the electronic brake balance controller, contained within ABS brake systems.

ABS Systems electronically control brake balance amd brake pressures, with the aforementioned solenoids. Using feedback from the VSS and ABS sensors in each hub bearing, to detect when the wheels lockup, the controller modifies brake pressures and balance.

Getting air into that unit, which is very possible, will provide false brake pressure and cause adverse effects to braking system.

http://www.brakeandfrontend.com/bleeding-abs-systems/

As quoted:"Typically, when performing wheel-based hydraulic repairs, no special steps are needed to bleed the ABS modulator. This is true as long as NO AIR allowed to enter the modulator. If air is allowed to enter, it may require special steps and/or tools to remove it."(bold caps are mine, obviously).

It further continues: "Case Study 2 Ė Modulator Replacement on a Delco VI Ė 4-wheel ABS system.

The Delco VI ABS system (See Figure 7) is a four-wheel, three-channel system that was used from 1992 until 1999 on a large number of General Motors FWD vehicles. In addition, the system was used on the ďFĒ series Camaro and Firebird and many Saturn vehicles.

The Delco VI represents the group of modulators equipped with bleeder screw(s). While this group isnít very large, there are enough of them to include in this article. As with any bleeder screw, these are placed at the highest point of the modulator, which on the Delco VI is at the solenoids.

Bleeding of the Delco VI will be required under the following conditions:


Modulator replacement;


Master cylinder replacement;


Solenoid replacement;


Gear replacement; and


System runs dry."

The Haynes manual is designed to teach newer mechanics the basics of vehicle repairs and care.

It contains information for both experienced mechanics and beginners, however, certain repairs, such as ABS braking systems and transmission rebuilds will be left out, as they tend to require specialty tools and or are extremely tedious or difficult for most at home mechanics.

The books are aimed at general populations, and should be appreciated as such, not to be mocked as FSMs will also recommend the Tech-II being used during ABS brake system services, per GM.

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"He checks the gas, and fills the oil....."

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Old 04-22-2018, 10:29 PM   #10
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1998 SC2
Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Quote:
Originally Posted by raymondjiii View Post
Interesting Nuc. With a 94 I assume Iíve got the older.
Yes, and as long as you can get fluid to flow from the rear bleeders the pistons are where they need to be. You can hear the system wind up everytime you start the car and get over 5-7mph, it is that odd sound you hear from in front of your feet.

The common rear ABS piston cuts the fluid off from the rear brakes to prevent a rear end break away slide if just pulsing the braks does not do the job.

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Old 04-23-2018, 08:28 AM   #11
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Well - my master cylinder has pretty much run dry. It should be interesting to see if I can get everything back to normal. I have the two rear bubble flare fittings off (that attach to the hoses.) Once these two fittings are on, I thought I would gravity bleed the rear calipers to start - and hopefully get something out. I am concerned about what to do with the brake pedal before doing the bleed. Normally they say to press up and down on the brake pedal four times before starting the process.

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Old 04-23-2018, 03:51 PM   #12
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Fill up the reservoir and then if need be draw a vacuum on the rear brake line bleeder with a mighty vac. Be patient. Don't be pedaling that master cylinder or that will be the next thing you replace.

If you actually get the rear lines cutoff by the ABS then the car goes to a good brake shop on a trailer or you find a tech-2 to home the pistons.

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Old 04-23-2018, 04:04 PM   #13
raymondjiii
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

When you are bleeding the master cylinder isn't that what you are doing: filling the reservoir and moving the piston back and forth.

I didn't follow what you said - fill the reservoir and "if need be" - how would I know if it's needed without applying pressure to the brake pedal? (Unless you meant the gravity bleed fails?)

I had one of those pumps that you can put on the reservoir cap and then open a bleeder valve - would that be a good idea?

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Old 04-29-2018, 05:10 PM   #14
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Default Re: Why does Haynes say do not bleed rear calipers without a scan tool

Very much to my surprise. I got brake fluid out of every bleeder despite having let the master cylinder run empty for over a week. I tried to use my mighty vac hand vacuum on one of the rear lines but barely got anything to flow. I had to try to hit the brake pedal to get fluid flowing.

Of course - I check the unions where I made the cut in the brake lines and sure enough, one of them leaks. I'm going to try tightening it and if no go then I'm going to try a bubble flare instead of double flare if I have enough tubing to work with. This bubble flare tool is so nice - you cannot screw it up - it's so easy.

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