ABS 1991 k1500

[Coloradochevyguy91]

So I have this posted in 1992 - present truck with no lick as of yet so I thought I would try my luck here.


I have air in my abs module on my dd I talked to a friend of a friend who worked for a gm dealership. And I told him the symptoms with the truck which are... extremely stiff pedal, long braking distance ( pretty much fills like my semi when loaded..), and slow rebound on brakepedal when applied.
So I talked to my buddies friend and told me when I replaced my rear wheel cylinders I let air into the system. Which made its way to the ABS modulator and told me there is no other way than to use a scan tool to bleed the system. So the next day I called my local GM dealership and I was quoted 278.00$$$! HOLY SHIT!!!!
So guestion is do I have to take it to GM or is there a way I can do this in my driveway?

 

[diesel4me]

Sounds hokey to me,but then I'm limited to 1980's and beyond as far as personal experience..
I have helped my friend at his shop bleed brakes on dozens of vehicles though,many in the 2000's with ABS,and we just use the pump the pedal,hold it,push it to the floor when he opens the bleeder,pump it up again method,and he's rarely had any issues doing it that way..sometimes I've had to start the engine to get power assist to bleed a few vehicles fully,but most of them we did with the engine off..

 

[kgblazerfive]

If, and I say if you let air in you can bleed it out. The only time you really cant is when you do a master cylinder. You have to do that before you connect the system into it.

 

[diesel4me]

The symptoms he listed do not sound like air in the system--air would make for a mushy pedal and maybe the need to pump the pedal more than once to get any braking action..

Sounds more like a failing brake booster or a faulty master cylinder,seizing up to me..or maybe the brake hoses are collapsed inside ,not letting the pressure reach the calipers or wheel cylinders,and restricting the return of fluid to the master cylinder..a dented steel line can do that too...

 

[Coloradochevyguy91]

I have replaced the master, and I did bench bleed it throughly. I will look into the brake lines in the front and if the booster was bad wouldn't I have a soft pedal?

 

[diesel4me]

Lack of power assist,or lack of vacuum to the booster,will usually result in a very hard pedal,and you'll find yourself "standing on the pedal" to stop...slow return of the pedal could be hydraulic issues with lines or hoses,or the booster may be defective internally..
I have seen more than one master cylinder that was NFG right out of the box also..

First thing I would check is to see if there is a good strong suction on the vacuum booster hose going to the intake--could be as simple as that hose collapsing,or the charcoal filter in line with it (that traps gas vapors and keeps them from rotting the rubber diaphram in the power booster ),is not clogged up...low vacuum = low power assist,and a hard pedal..
Then check the rubber brake hoses and steel lines for free passage of fluid--you can use an air gun to blow thru them to test them too--sometimes they will collapse inside and act as one way check valves,or simply plug solid and let no pressure reach the calipers or wheel cylinders..

 

[Coloradochevyguy91]

Lack of power assist,or lack of vacuum to the booster,will usually result in a very hard pedal,and you'll find yourself "standing on the pedal" to stop...slow return of the pedal could be hydraulic issues with lines or hoses,or the booster may be defective internally..
I have seen more than one master cylinder that was NFG right out of the box also..

First thing I would check is to see if there is a good strong suction on the vacuum booster hose going to the intake--could be as simple as that hose collapsing,or the charcoal filter in line with it (that traps gas vapors and keeps them from rotting the rubber diaphram in the power booster ),is not clogged up...low vacuum = low power assist,and a hard pedal..
Then check the rubber brake hoses and steel lines for free passage of fluid--you can use an air gun to blow thru them to test them too--sometimes they will collapse inside and act as one way check valves,or simply plug solid and let no pressure reach the calipers or wheel cylinders..

Awsome I will check my booster later today and report back to you. Also would bad calipers have anything to contribute to this???

 

[diesel4me]

If the calipers were seized up completely it would make for a very hard pedal and little or no braking action,but it would be very unusual for them to seize that tight-(maybe if the vehicle sat "forever" )---usually they will still apply pressure to the pads and not release,and the brake pads will smoke,and want to lock up the wheels..

 

[diesel4me]

If the calipers were seized up completely it would make for a very hard pedal and little or no braking action,but it would be very unusual for them to seize that tight-(maybe if the vehicle sat "forever" )---usually they will still apply pressure to the pads and not release,and the brake pads will smoke,and want to lock up the wheels..

 

[Keitha]

Not sure if this will help, but while bleeding your brakes: There is a small rubber cover on the ABS proportioning valve - under the cover is a small button. Make sure you push that in prior to bleeding. I think that resets the check valve.

 

[paratrooper307]

Yeah, the scan tool thing is some hooey-although it is recommended on newer vehicles where computers actually have the ability to control such things through an OBDII connection. Modern computer-controlled vehicles have the ability to command various functions, from EVAP purges to ABS pump bleeding and more-but the computers on a 91 are pretty rudimentary and don't have such abilities. Even on the newer vehicles, you can get around the "command purge" issue by using a pressure bleeder. About 15 psi and above will force open the valve and achieve the same result. Agree with the others though-hard pedal is not a symptom of air in the system.