Hercules

I recently had to replace several brake lines and the wheel cylinders (I used new GM 1 ton wheel cylinders) after having a line blow out. The brakes work great with one exception that I'm noticing. When I drive locally, they work perfectly...or at least nothing is noticably wrong. When I drive longer distances on the highway between having to apply the brakes, and usually when having to slow down to a stop from higher speeds (like 70 mph), I get a high frequency pulsation that feels like I'm running over shallow rumble bumps and can be felt through the brake pedal too. The problem goes away after that. The pulsation does not feel like a warped disk or drum and doesn't appear to slow down with the speed of the truck. Tonight, I noticed it did it from 55-60 mph. Also, most the time it doesn't do it at all from any speed. I'm thinking because I didn't bleed the whole system (abs unit, RWALvalve, combination valve) that an air bubble is trapped somewhere before the lines to the wheels. I'm hoping to bleed the whole system from the master cylinder to the wheels and see what happens. What do you think could be causing this?

I used my brother-in-laws Snap On scanner yesterday thinking that maybe there was a code saved from when the original line blew. I thought that if there was a code and I cleared it that it might fix the problem. The only code I found was 4-wheel abs code #78. I could not erase this code and am wondering if it is some type of check code. With that said, does anyone know what code #78 means?

Thanks for the help!

seahawkdodge

Well, I would start with bleeding the entire system, and go from there. I have no clue about the code though, hopefully someone else will chime in here.

Hercules

Well, I bled the entire brake system and the problem still persists. I did get a tiny bubble out of the RWAL valve when I bled it, but, it may have been air that came out from between the line and the hole in the nut that the line goes through...I can't say for sure if it came from inside the valve because it was so small. I ran another pint of new brake fluid through it when bleeding just to make sure as much of the original fluid was gone...that makes 3 pints of new fluid from the start of this. I do know the anti-lock brakes do work. I slammed on the brakes and could easily tell each wheel that would lock then unlock and the truck stopped well. I have some unexpected time tomorrow, so I'm going to my brother-in laws machine shop and putting the drums on a brake drum lathe to make sure they're trued up. There's always been a very very slight out of round condition to the drums, but, nothing that I have ever seen cause a vibration like this, not to mention it's never been noticalbe before when driving. Today is the first time I can say that the vibration appeared to slow with the vehicle, but, I also wasn't having to stop from 70 mph in a short distance/time. I'm wondering if the GM 1 ton wheel cylinders are hypersensitive to this and are amplifying the condition of the drums. I know the disks in the front are true, so, I've ruled them out. The front calipers don't appear to be binding and the pads are in great shape with probably about 90% life left in them. The rear shoes are new. I'd still like to know what code #78 means to 4-wheel abs too. On the up side, I'm getting pretty good at bleeding the brake system.

GearHd6

Are your rear brakes adjusted up to tight causing the drums to heat up and warp due to the higher speeds for a longer period of time?

Hercules

No, the rear brakes are adjusted so that you can just hear the shoes and drum scrape when turning the wheels by hand. At least that's how I adjusted them last time. I'm guessing you probably can't hear them scrape any more after putting over 200 mile on since I replaced the lines and adjusted them last week. Tomorrow, when I take the truck up to my brother-in-laws machine shop, I'm driving the truck straight in and on to a lift as soon as I get there. I was planning on going over all 4 wheels while some heat is still in them to look for anything out of the ordinary before pulling the rear drums off to lathe. The only thing I'm worried about is if the drums don't have enough thickness left to lathe. I'm guessing they've never been cut, but, I don't really know since I'm the second owner of this truck and I didn't measure them when I had them off last time. I'll find out tomorrow when I pull them. Also, I'm going to pull the front calipers off to clean and grease just to rule out one more thing. I don't think anything is wrong with them since the pads are wearing evenly and don't appear to be sticking, but, while I have it on a lift, I may as well.

Hercules

I lathed the drums today. So far, so good. I don't think they were out by very much. I only had to remove .005 on one and .007 on the other to get them trued up, then did a finish pass. Everything appears to be working great now. I'm thinking that because I removed so little, the actual problem was the surface of the drums were glazed. Even the surface of the new shoes appeared to have glazed over, probably caused by the drums. I scuf sanded the shoes to remove the glaze there too. I think the combination of new shoes and glazed drums caused all my head aches. Hopefully, this is done now.