Does Abs Dissable Dual Master Cylinder Operation?

[corvetteguy]

A few weeks ago my 99 Savana and I were going down King Hill.

Near the middle of the 1/2 mile hill (ending in a "T"), my pedal went

down to the floor. Pumping had no effect pedal went to the floor.

Parking brake seemed ineffective. Low gear helped but I wound

up sliding through intersection making a right.

 

On lift I found the rear brakeline was leaking along the frame near

the gas tank. I traced the line and it went to an ABS unit under the

driver.

 

WHY DID THE FRONT BRAKES FAIL TO STOP ME?

 

DOES IT HAVE SOMETHING TO DO WITH ABS?

 

when bleeding the new line with a vacuum pump no fluid/air

would come out until I pumped the pedal also.

 

DOES THE ABS PREVENT BLEEDING?

 

I notice the dash warnings never show an ABS ICON WHY?

 

Help.

[corvetteguy]

That is what I thought

 

Was able to bleed line the old fashioned way with

someone pumping the brake.

It is working as before However, I worry

the next leak will cause the same drop in pedal.

 

Maybe a preemptive changing of the Master

Cylinder will make me safer.

[Jsdirt]

Find a dirt, or a sandy road & lock up the brakes several times to engage the ABS motor ... then let it sit for a couple minutes ... then LIGHTLY touch the brake pedal a few times. Sometimes this allows bubbles to rise into the master cylinder, removing any trapped air.

 

If that doesn't work then lock 'em up a few more times, then bleed conventionally. Eventually the air will come out.

[redvett]

Your truck has ABS but the light should cycle on when you turn on the ign. Possibly someone removed the bulb.

[corvetteguy]

http://www.autoshop101.com/forms/brake02.pdf

 

 

I like the explanation.

 

Now, do you have anyn lessons on ABS?

 

Thanks

[markw8]

I can't say why you lost all braking but most systems aren't split frt/rear anymore. They are diaginally, left frt, right rear. If you have air in the abs system, you might have to bleed it while it is cycled with the correct scan tool.

 

Mark

[redvett]

RWAL was used on 1988 to 1994 GMT trucks and ABS was used on SUV's Suburban & Tahoe starting In 1992. RWAL is REAR WHEEL ANTI LOCK and ABS is ANTI BRAKE SYSTEM.

[corvetteguy]

I can't say why you lost all braking but most systems aren't split frt/rear anymore. They are diaginally, left frt, right rear. If you have air in the abs system, you might have to bleed it while it is cycled with the correct scan tool.

 

Mark

When purchased I did not think the van had ABS, also there is no

dash ICON ABS.

In fact there is only 1 line going back to the rear axle where it

"T"s to go to both wheels.

Maybe it isn't ABS? Than what is the unit on the frame under the driver?

 

Is there a quick test to see if the master cylinder is working correctly?

Like loosen one side and pump brake?

 

Thanks

[8.1HD/ALLISON]

Probably has ABS, Rear is usually one circuit, Front is split left and right. You probably need a scan tool to actuate abs motor to bleed brakes. Front brakes should have worked.

[Doug_Scott]

It was pretty common in the early days of ABS to only have it on the rear brakes on pickup trucks and vans. The traditional split master cylinder had two unique brake systems that would use the hydraulic pressure from one part of the master to apply the second part. If you had a failure on one of the two parts, there was a failsafe method that would apply the brakes on the system that did not fail. Basically the pedal would push the rear piston in the bore of the master cylinder, and that would in turn apply hydraulic pressure to the piston directly ahead of the piston being applied. If one system fails, the two pistons would physically touch to apply the brakes on the system that did not fail. It is a very simply system, but a bitch to explain with just text. See this pdf for a good explanation...

 

http://www.autoshop101.com/forms/brake02.pdf