I just replaced the master cylinder on a 1994 diesel K5 Blazer w ABS. It seemed like no big deal at first - I got the new cylinder bench bled and ready to go, then unbolted everything and started to pull the old cylinder and drop it into a bucket when I realized the brake lines going into the ABS control box were in the way of the cylinder sliding out nicely. (If the thing was just mounted a half inch further forward there'd be no issue - with these trucks Chevy sure was expert at taking easy jobs and making sure to design in needless serviceability headaches to make them a pain!)

In retrospect I should have unbolted the control box bracket and wiggled the thing enough out of the way to get the cylinder out. But since at this point I had fluid dripping everywhere, I hurriedly decided to just disconnect the aft most line from the ABS control box and move it out of the way so that I could get the master cylinder off the hydroboost unit. I was planning to fully bleed the system anyhow, so getting a big air bubble between the proportioning valve and the ABS box didn't seem like a problem.

Only then did I realize that I hadn't had to bleed the brakes on a ABS equipped vehicle before and quickly found out that it's not as easy as in the olden days! Has anyone done this job? To do it properly I understand you need an ABS scan tool - any recommendations for this? (I assume it's a different tool than the ALDL code reader.) I wanted to do a full bleed, but since the air is all between the master cylinder and the ABS box, are there any shortcuts to avoid the need for a full bleed and the associated tools?

Thanks!

 

Or huh, the lousy Chilton manual says I need a scan tool, but maybe only for later years than the '94 as some stuff on the web seems to imply?

 

I've done brake lines on about everything ever made and have never had to bleed a abs. Usually I just try to bleed it normal if that don't work or brakes feel crappy ill then bleed the master at the lines. Actually the other day I put a master on and had to bleed at the lines going down to the abs on a ford escape. So try that. Punp it up and crack the lines at the abs and master and see if you get air out

 

I'll give that a try and I bet it will work. I get the impression that ABS bleeding isn't an issue unless air actually gets into the ABS box workings from air being in the system when the ABS cycles upon wheel lock, or if the box is new. Someone correct me if I'm wrong about that. I've seen some posts around that indicate that people definitely have had some problems when air gets in the ABS for whatever reason, and a frequent recommendation is to bleed normally and then slam on the brakes on grass or gravel to cycle the ABS and shove the air into the main lines where you can get to it.

I've got to rig up a pressure bleeding cap for this thing and will report back... I do still wonder whether the infamous GM Tech 1 scan tool is needed to do a proper ABS bleed on this thing, as some of what I'm looking at seems to conflict. But hopefully it won't matter!

 

Yep, pressure bleeding the lines between the master cylinder and ABS box did the trick. Good thing too, since when I went to bleed at the wheels for good measure and to purge out old fluid, three out of the four bleeders just sheared off rather than coming loose, despite penetrating oil! I'll have to get after those with screw extractors some time soon, but for now the thing's back on the road.

 

if they are broke off closed your better off just leaving them alone unless youre really bored. ive never been able to get them out after they break. at work lately if the bleeders dont seem like they are going to come off i will heat them up with the torch, that works half the time. also if a bleeder brakes you can bleed the brakes at the line