Hate to bring back this really old thread. Fast orange, did you ever fix the pulsating? I am in the same boat as you. New rotors pads drums master and still does it. havnt bypassed the ABS yet. Its really getting on my nerves.
If it pulsates all the time, it's very unlikely to be the ABS unit. If you can bake at highway speeds and it pulsates, it's not the abs unit.
Pulsating brakes without the ABS engaging (you have to be almost stopped for abs to activate) can be caused by several factors.
The most common is warped brake rotors. But this isn't the only cause.
You can have rust under the brake hat mounting surface making it not run true. You can have sticking caliper pins. You can have worn caliper mounting pads. Crap brake pads can cause it (ie: poor fit in the mount). You can have problems with warped drums. You can have oil on the shoes. I've even seen a unitized bearing cause brake pulse because it was not running true (cheap parts will burn you every time). While I've never seen it, I'll wager you could also end up with a similar issue (IE: not running true) on the rear axle.
Just so you're aware: new rotors doesn't mean they won't "pulse". If they have too much runout, they will pulse. It's more common than you might think. New rotors can warp on the first hard stop also, another thing that is more common than one would think. I blame thinner rotors sold by manufacturers/jobbers in an attempt to save a few bucks and boost profits. There was a time when turning rotors was a common practice. Not anymore. Most new rotors are manufactured at (or very near) min thickness. They sell it as saving weight and increasing MPG, but it's really about increasing profits. There is some incremental MPG gain, but it's nearly statistically insignificant to the individual driver. But, that 5-10 cents saving in iron per rotor turns into real profits over a million unit run. B@st@rds.
Personally, if I have a hard stop or am towing heavy, I stop and then roll ahead slightly. About one rotation of the wheel if I can. I will even do it if I have braked from highway speed, which is also classified as "heavy braking". This helps to distribute the heat that has built up and helps prevent rotor warpage where the brake pads are "death gripped" on the rotor. The pad insulates that spot and holds the heat in while the rest of the rotor is cooling. Warpage is the result or even localized hardening of the rotor, which can also make it pulse. You need to have room to safely roll enough though....sometimes you just don't have that choice....
If you want to see if your ABS unit is part of the problem, just unplug it. This causes it to go into bypass and it becomes (essentially) straight lines. However, they don't cause "pulsing" in brakes during normal operation. In normal operation (IE: not locked up) it's just a passenger and is in bypass giving you full control over the brakes. What an ABS can do to normal braking is a soft pedal feel. This is due to trapped air. If it is not bled properly it can introduce air into the system and cause a soft pedal. It goes something like this: you bled the brakes, it feels good, the ABS unit has air in it but you didn't get it out in a normal bled procedure, you drive the truck, feels good, then the air comes out of the abs unit into the main system, brakes get soft, bleed the brakes, feels good, you don't get the air in the abs and so on. This is why newer vehicles have to have the abs unit functioned with a scan tool while bleeding; they are more complex than our simple "accumulator and valve" abs.
Here's a pic to maybe help understand it:
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you can see where the isolation valves essentially remove the ABS from the brake system when you unplug it (they are closed when ABS is not activated). It bypasses nearly all of the passages and accumulators in the abs unit, making (for all practical purposes) the same as a piece of brake line.
Be aware that an ABS unit that activates under 5-10 MPH for no reason can actually be very dangerous. It increases your braking distances if you don't just mash down on the brakes pedal. It takes away your threshold braking (what you are essentially doing rolling up to a stop sign, coincidentally; when you are below 5-10 mph) since it thinks you have locked up the wheel. The GMT400 chassis has a somewhat known problem with this when they get older. What happens is rust inside the hub gets stuck to the sensor or the tone wheel (the sensor is a magnet). When this happens, the ABS sees not alternating pulse and thinks the wheel is locked up. It then "does it's thing" and goes into it's lock/unlock procedure. The cure is to remove the senor and clean the "rusticles" off it or replace the unitized wheel bearing assembly....
