First Brake Flush. What did I do wrong???

[James]

Hey
I just did my first brake flush on a 1999 GMC Safari AWD. I followed all the correct procedures and made sure no air got into the system however i did not flush the front wheels because i had sheared off the bleeder on the right side so I just left both of the front wheels alone. I plan on changing the callipers anyways.

The problem I had when I test drove it was that the brake pedal would kick back slightly under normal braking conditions I don’t know what could cause this? Also, the fluid I took out was very old and based on how much rust was on the bleeders I doubt the fluid was changed regularly by its previous owner.

Topic

[WOTStang]

Are you getting a noise associated with this kick back? Or is it just normal quiet operation as it does it? Does this vehicle have ABS?

It’s 1 of 2 things off the top of my head.

1. There is air trapped in the system and your getting a bubble trapped somewhere and the fluid is suddenly filling the air cavity (air can be compressed, liquid cannot) so your getting a kickback when this happens.

2. You have air trapped in the ABS HCU due to air in the system. Which in turn is going to lead to the same symptom as above. Im not 100% positive that air in the system will lead to an ABS operation event, as this is mostly controlled by wheel speed sensors.. however i have witnessed some crazy operations due to air in the hydraulic control unit with stuck open/closed dump/isolation valves.

My best advice to you at this point is to replace the calipers and do a full re-bleed with the proper sequence and flush (and new fluid.. dont use a previous open bottle) Also take a good look at your brake hoses for any breaks/tear.. even if there is no fluid associated with them.. anytime a brake hose has a tear or crack in it, it is considered compromised and should be replaced. Do that, and it should take care of your problem! 🙂

[Ingvar]

You realize you have X-shaped dual hydraulic lines and bleeding only rear calipers is pretty much like bleeding half the system? Lines are crossed in X manner so that when one side fails, you still have one front and opposite side rear brake functional. Safety feature.
Kick back in the pedal is normally, normally, ABS response. I had 99 Silverado, I know that response very well.
ABS block has to be bled additionally to normal bleeding. This is done by rapid acceleration to about 50mph, then both feet on the brake pedal aggressive braking. 2-3 times in a row. That bleeds air out of ABS module.
Bleeder screws in caliper are replaceable. No need to replace entire caliper because of that. But you do have to bleed both front and rear, for reason above-mentioned.

[Ingvar]

FYI, this is your brake system:

http://www.aa1car.com/library/abs_kelseyhayes.htm

[James]

Thanks I had no idea what x shaped dual hydraulic brake lines where :oops:. But how could there be air in the ABS system I never allowed any air to get into the lines or through the master cylinder.

[twiggy]

It’s possible that your issue wasn’t caused by your recent work bleeding the brakes- with new fluid you might just now have the ability to feel the pulsing in your pedal.

If you have an x shaped system, then flushing just the rears should have gotten most of it out.

[Andrew Button]

We used to talk about this being written on the school wall, flush twice, its a long way to the school cafeteria.

Maybe thats it

[Author]

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