Previous owner installed a new master cylinder because he thought it was bad. I'm begining to believe it is the power brake booster. Or maybe linkage is wrong or missing? I press the pedal till it stops (but that's not all the way to the floor) and the truck slows to a stop. Doesn't even come close to locking any wheel up. I pulled apart the brakes front and rear. I replaced the front calipers and a rear axle shaft seal leaking on to the shoes, put everything back together, bled from rear to front. Same result. Booster? Bad/missing/out of adjustment linkage?

1994 GMC S15 JIMMY 4.3 CMPI auto 4x4 abs

 

I have ran into issues like this a few times. Most of the time, it was an improperly bled master cylinder. Some master cylinders can be a royal pain to get bled properly. If you see a spout of fluid in the reservoir when the pedal is pressed, then there likely is air in there.

 

Also I just learned there is a jb5/jb6 brake system and a jb7 which uses a different master. AND yes there is a spout of fluid. i think i shall take the unit off the truck and re bench bleed it. Is there anyway to tell which master cylinder he installed? Maybe there is a part number stamped on it.

 

You'd have to check around the part to see if there is any identifying marks on it.

You don't have to remove the master cylinder completely unless you are working by yourself. Taking some short sections of brake line with the appropriate connection on the end to seal the outlets of the MC, bent to direct the fluid back into the MC reservoir below the fluid line will keep things contained while you or an assistant pumps the brake pedal.

 

had a customer bring a s10 into the shop, i think it was a little newer 98ish maybe same kinda issue. he replaced rear calipers and still nothing. it was really weird. when i bled the rear brakes with someone pumping no fluid sprayed out it just kinda gravity bled out. i ended up going up front and bleeding it at the master then at the rear output line coming off the abs module and got some air out of that thing. then went back to the rear calipers and got alot of pressure. so who knows if theres some kinda safety device in there or it just dont work when there is air in it

 

Yeah I remember something about a special tool that the dealer connects to the ABS controller and uses to purge the ABS solenoids and that we cannot do that ourselves as the ABS does not fire when we are doing a manual bleed. I think this is the problem when doing brake work and the fluid is allowed to drip dry getting air into the ABS stuff. Ive just always been mindful to check the fluid and let it drip till I put things back together so that it never drips dry. I still use like 2 bottles of bleeding before Im satisfied with the flow out the rears on mine. I have never tried cracking the bleeders on the ABS controller though to see if I can get air out of there.

EDIT

You could try this method of opening the ABS controller bleeders.
http://www.auto-maintenance-repair.com/braking/440.html

 

Since we are here talking about brakes. Would a loose bleeder screw on the abs unit cause my abs light to come on? (im going to say yes, but want to verify)

Noticed brake fluid on the abs unit and sure enough, the screw was finger loose.

Bidding on an obd1 & obd2 scanner so hopefully I win it. Id love to have a scan tool of my own!

 

My Snap-On MT2500 was the best thing I've ever bought for a car. I got it for $200 from someone on Craigslist who needed rent money. A few extra cables and cartridges on eBay and I was all set. Now I can always read the throttle position sensor on my cars. I also like to take it along to check out used cars before buying them.

 

That sounds like a smart idea. Wish I had one before i bought the blazer, although I really like this thing...minus everything needing fixed, which is just about everything from rear hatch, door handles, inside and out, all the way up to the windshield needing replaced...lol

 

Don't feel discouraged choover82. Mine needs alot of work too. Been too busy to work on it.

The power brake booster holds vacuum for a long time. Engine off, pull hose off after a few minutes, big rush of air. Also, I pump the brakes with the engine off, and then hold the brake when i start it, it drops quite a bit. (also a good sign i believe) I am going to just purchase a new master. I just don't know which master to purchase. does it have two different size bores for the front or rear? I received the part number from GMC with my vin #18013433 but i think rock auto lists that W/power brake booster as a unit????