1998 Suburban brake issues

[Coal-ka-nut]

Hey there Chevy Forum...Got this awesome 98 Suburban that is giving me some issues with the ABS system.
When you apply the brakes , especially when you just start the truck, it engages the ABS, and pulses on the brake pedal. My wife is frightened to use it with this issue, so I pulled it out of service.
It never gave me a single problem, other than a fuel pump issue since I have owned it, and that is 14 years now. I have 117000 miles on it, and it is a very nice unit still.
I would like to give it a go, thinking that my master cylinder is going weak. Does anyone think it could be in the control manifold with all it's wizzardry inside? Is this something the average guy could attempt on his own? I had done front brakes about a year ago, and they are fine. Rears are also good to go. So why the mushy pedal?
Help me!

 

[Spriteman]

I believe that the ABS system does an system initialization at startup including cycling all of the valves.

Does the ABS activate every time you push the pedal or just on startup?

 

[VanGo]

Have you checked your codes?

 

[zora]

A little confused, you first state you wonder why the ABS is cycling when first started but than as almost an after thought you mention "mushy brakes" in the last sentence. If you have a mushy pedal there is something else going on here. Did you check the fluid level in the master?

 

[VanGo]

A little confused, you first state you wonder why the ABS is cycling when first started but than as almost an after thought you mention "mushy brakes" in the last sentence. If you have a mushy pedal there is something else going on here. Did you check the fluid level in the master?

I was confused by that description also. Big difference btwn ABS pulsation and 'mushy' brakes.

 

[Coal-ka-nut]

Well thanks for taking the time to help me. It is just like I said. Say you are pulling into a parking space, and the pedal starts pulsing like the ABS is engaging. And then at times the pedal is soft or mushy feeling. Not quite like when you bleed the brake lines, but weaker than it was when it was normal.

I did check for codes. Nothing came up on my Scan Tool.

The fluid level is normal, and clean. I did the front brakes last spring, and changed the fluid out in the system.

If the master cylinder would be worn, and cause a lower brake pressure, do you think it would activate the ABS system? Not much help in the Hayes Manual on this topic. Does this information help?

 

[Spriteman]

I think ABS codes are not able to be read by all scanners - they have to have ABS capabilities.

It seems to me that the wheel sensors may be a good place to look. The ABS is triggered when braking the wheels are turning at different rates and the ABS tries to compensate by allowing the slow wheel to catch up and then reapplying the brake.

Check the connections at all of the wheel sensors for a quick look.

You could also try to track down an ABS scanner - I believe there are diagnostic tests on the scanner that may reveal some insight into the problem.

 

[VanGo]

'Mushy' pedal usually means air in the system. I would try to bleed it again first. A bad master cylinder will typically leak fluid around the pistons and eventually bottom out on the floorboard. It's possible that the ABS is compensating for the brake system problem. A good code reader would tell you if something is wrong with the ABS or if it is just doing its job as Spriteman mentioned.

 

[Coal-ka-nut]

Thank you everyone for the comments. I will check out the items identified here and bleed the system again. I have no codes showing, and no leaks. New front hubs also along with brakes.