Hercules

In the process of chasing a brake problem, I noticed that the pistons for both front calipers are very hard to push in. When I say "push in", I am refering to a situation like when you are replacing front brake pads and you have to push the caliper pistons in so the new pads will have enough space between them to slide back over the rotor for reinstall. I have never seen/felt this much resistance from any caliper on any vehicle. It feels as if they start compressing then build more and more resistance. When I pulled the calipers last week, I had to use a small c-clamp to compress the calipers to clear the rim of rust that was forming on the rotor and I was only able to move the pistons less than 1/8 in. The thing that's bugging me is both calipers are like this. I'm wondering if this is a coincidence that both are doing it and the calipers are going, or, if this has something to do with the ABS since the truck has 4-wheel ABS. Tomorrow I'm going to pull the calipers again and try compressing them with the bleed screw open. Does anyone have any thoughts about this or advice on what could cause them to have such large amounts of resistance?

BTW, the brake problem I'm chasing is a vibration/pulsation that can only be described as is the ABS went hyperactive. I can not tell for sure if it's coming from the front or the rear. It feels like it shakes the whole truck. The problem developed after the last brake lines blew and replacement of the rear wheel cylinders with the GM 1 ton upgrade while replacing the lines. Like I mentioned before, the problem is intermitant and doesn't appear to have any ryme or reason to when it does it. It's only more noticable when slowing from high speed, but, can be felt when stopping from low speeds. It doesn't matter if the truck is empty or has a ton of rock in the bed. Other times, it's like nothings wrong. There is one exception. I have recently noticed when I pull a long hill at anything above 55 mph, maybe 1 mile, then catch a red light just after I crest the hill, the problem will be more noticable and violent. There are no codes for the ABS either. Any thoughts on this are appreciated too!

dozer12216

With the bleed screw open is way you always compress. Do not force bad fluid back into master cylinder or throuh the abs system.
Do not attempt to compress, just go get a pair of reman calipers and be done with it. Why do you think you have a vibration???

Hercules

Dozer, all the fluid is new, from the master to each wheel. I flushed the whole system when I replace the blown lines and wheel cylinders, and again while chasing this problem. Mostly to insure I didn't get a bubble trapped somewhere and secondly to insure all the old fluid was out. It just strikes me odd that both calipers would behave the same way, which makes me wonder if it's really a problem in the abs system. I was figuring if I compress the calipers with the bleeder open and they move freely, then I effectively would rule them out as the problem. Is there some other way to test the functioning of the abs, because when I hooked my brother-in-laws snap-on computer up to it, it's saying everythings fine and no codes? The only other thing I can think of are the GM wheel cylinders. They are very touchy sometimes, especially in crawling traffic. I was also wondering if since the rear wheel cylinders are actually putting some usefull stopping power to the rear now, that maybe there's less of a demand placed on the front and it's causing the old calipers to function irratically. I just hate throwing parts at something to solve a problem. Although, I guess it can't hurt to start fresh up front too.

bmoeller

My guess is the ABS is doing its thing. The bigger wheel cyl would have the rears working harder, and increasing the chance of wheel lock up. Try backing off the rear wheel adjustment a tiny bit. You might have it a little tight.

And, as stated, you should NEVER try to put in the caliper pistons on an ABS equipped vehicle without loosening the bleed screw.

Number47

as stated before dont push the old fluid back into the system open a bleeder but if you let the caliper hang from the hose you can damage he hose and it will start to act like a check valve and leave the brakes applied. but if you open the bleeder and it still really hard to do (like you can barely get it with channel locks and your thinking about a c-clamp) replace the caliper they are around 70-80 with new brake pads, they call that a loaded caliper

infidel

I've had to replace two 4 wheel ABS units because the owners forced the calipers back without the bleeder open. New fluid or old, it doesn't matter.
They aren't cheap, around $800

Hercules

I got the chance to talk with a mechanic that moonlights at my brother-in-laws machine shop. He's been a GM and Dodge mechanic for 25 years combined. I asked him what he thought, too. After hearing everything that I've said here, he didn't even hesitate saying it's the rubber brake lines causing the compression problem. He claims their degrading inside and restricting the flow. He said he sees this problem so much now that every time they replace calipers now, they replace the rubber lines too...especially on vehicles this age. He told me to do several things...replace the rubber brake lines, replace the calipers while doing the lines since their cheap enough, and make sure the front abs sensors don't have any rust screwing up the sensor readings(something else he said he sees alot that causes the hyperactive situation I mentioned earlier). He looked over the abs system and did a few things, unfortunately I had to take a phone call at this point and can't say what all he did, but, he's convinced the abs is fine. Anyways, hopefully I'll get the chance before the end of the week to do what he recommends and post back the results.