Need help on brakes

[c5vair]

Hello folks, I have a 1993 Chevy crew cab 1 ton diesel with new wheel cylinders new brakes in the rear no oil in the drums and it's also got a new master cylinder new hydroboost and everything was fine until lately if you just tap the brakes the rear brakes lock up it's like there's no fluid going to the front brakes the master cylinder is full I pulled drums in rear agin they were clean, is there something going on with the front abs system? There is no abs light on

[DennisG01]

Does this continually happen? Or is it just once in a while - like maybe after a rain or if it's been sitting for a bit? Does it go away after a few miles of driving?

[c5vair]

I am not able to even drive it right now because of the brakes. So it's not after a rain or anything that makes it act up.

[OIL BURN]

I 'll try this again, posted 3 paragraphs, they suddenly vanished, computer gremlins.There is a chance you may have air in the front brake lines,so when braking air is being compressed,no pressure to calipers although the ABS controller senses some pressure to front brakes and senses everythings O.K.. Rear lines are getting brake fluid with little or no air so they are doing all the braking exhibiting lockup. This condition can vary depending on temperature of fluid and any air bubbles traveling around the ABS hydaulic unit.

From what I've read ,you should be able to replace wheel cylinders and bleed the system as you would in a non ABS system,but,to be sure an ABS controlled system should have one of those tech tools pluged into the ECM plug to activate and cycle the ABS hydraulic unit during the bleeding process to eliminate any air that may have entered. Unfortunely,dealerships have them,call around and see if a local brake shop has one.Flushing the ABS hydraulic unit is good maintenance and air free new brake fluid throughout the system is a plus.

Front to rear brake pressure application is automatically controlled by ABS hydraulic circuits and ABS controll module fed information from wheel sensors voltage changes during rotation when braking.After bleeding, scan system again,clear any codes, the tech tool should do this.As a side note,some vehicles have one sensor for both rear wheeles located on trans extension housing.

On non ABS systems,a proportioning valve accomplishes this front / rear brake pressure application so bleeding the system is simple.Personally,I prefer Non ABS,I like a hard pedal not a mushy one,but I've had training on proper braking on wet and icy roads. Black Ice, forget it, no ABS system will stop the sliding.Overall,ABS is good for alot of folks who panic stop.

If all return springs and hardware are of proper type and assembled correctly,shoe to drum clearance correct during assembly,and self adjusting mechanisms correctly installed, very important,then go for a complete brake fluid flush.Other than a problem with the ABS hydraulic unit,this should fix it,if not, focus on the hydraulic unit. It's a process of elimination.

If anyone can add or correct what I have written,please do so,so we can help this member get his rig on the road.

THANKS

OIL BURN

[c5vair]

Thanks for your help I live in northern Michigan and I went to texas to have a rust free truck to build, everything in the drive train is rebuilt so I don't drive it in winter, i have some time to figure it out, thanks for your ideas after the holiday seasons maybe I will have enough money to take it to the dealership

[convert2diesel]

May not need the dealer. Next snowfall or freezing rain, before the salt trucks get out, take the truck out and really exercise the anti-locks. Bring it home and bleed the system again. Wash truck.

Works for me.

Bill

[Nobby]

Yep ABS will need bleeding, what Bill said poor mans ABS bleed equals find a slippery surface and brake hard to activate ABS. I prefer a dry gravel road for my slippery surface Then bleed system again all four corners, passenger rear first. Also hammer tap front calipers during bleeding as it removes air bubbles that have a habit of sticking to the piston.

Cheers
Nobby

[a5150nut]

Also hammer tap front calipers during bleeding as it removes air bubbles that have a habit of sticking to the piston.

Now that is something I need to tuck away in the old memory.:D

[More Power]

Your 1993 has a very-very basic ABS. It's only rear ABS. There are no front wheel speed sensors

There is an electronic module located beneath (attached to) the master cylinder that receives input from the vehicle speed sensor, which electronically controls hydraulic brake pressure within the master cylinder that is sent to the rear brakes.

As long as the VSS is indicating some forward/backward motion of the truck, the master cylinder will proportionally apply hydraulic brake pressure to both front & rear brakes. If the VSS indicates no vehicle motion, the RWABS on the master cylinder will dump pressure that would normally go to the rear brakes, which in theory will prevent rear wheel lockup.

Bleeding these brake lines is as simple as it can be. No fluid travels through any ABS control system.

[c5vair]

Thanks for the replies I will try bleeding the system

[a5150nut]

Is your speedometer dropping out? Going to 0 when your problems happen?