Author Topic: brake master cylinder  (Read 5829 times)

Offline Lt.Del

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brake master cylinder
« on: June 27, 2006, 06:48:00 PM »
Ihave a situation with my 91 sub.

The brake warning light comes on intermittently.  I know it is not the parking brake pedal coming down. She's all the way up and stays there.

The light will come on for a few minutes, then cut off for a while.  It keeps doing thisl.

The brake pads, rotors, fluid, etc are just fine.  I have noticed no difference in the brake effeciency--they work just fine.

Someone told me there is some kind of float in the master cylinder and it is common for it to malfunction and turn on the warning light.  Is this correct?  

If so, do I have to replace the entire master cylinder?

Again, the brakes seem to work just fine but this little light is annoying and when my wife rides in my sub, she always fears the worse..."you don't have brakes...".

When the sub goes up for state inspection, it won't pass as it is, whith the little light coming on. Gotta find the problem.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4


Offline roundedline

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Re: brake master cylinder
« Reply #1 on: June 28, 2006, 10:29:00 AM »
If you pull back on the brake pedal will the light come on?
There is a switch that is on the brake pedal under the dash.  If you brakes are working ok, the switch may need adjusting...

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project


Offline 1976Scottsdale

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brakes
« Reply #2 on: June 28, 2006, 12:26:00 PM »
Check the wire that runs to the proportioning valve, make sure it has a good connection.  If that checks out, bleed the brakes and see if there was air near the valve, mine was doing the same and bleeding it made it quit.


Offline Lt.Del

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Re: brakes
« Reply #3 on: June 28, 2006, 07:43:00 PM »
Good ideas fellows.  I'll try and bleed some fluid out...I would just think that if it was air in the lines, the pedal would be a little spongy, and it isn't. But i guess there could be a little air in just the right spot.

I'll try pulling up on the brake pedal and see if it comes on/off.

This mechanic guy said it may be a float or something in the master cylinder...i'd never heard of such a thing before. The master cylinder seems to be just fine as far as operating the brakes.

thanks, i'll keep you posted.

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4


Offline VileZambonie

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Re: brakes
« Reply #4 on: June 28, 2006, 10:11:00 PM »
this is what can turn on that warning lamp...

a) the bulb check (key on engine off)
b) the differential pressure switch in the combination valve
c) the low fluid level sensor in the reservoir
d) the parking brake switch
e) a short to ground after the bulb in the cluster ckt
f) an RWABS fault

The easy thing is all of these ckts simply ground the bulb. Unplug one component at a time and when the light goes out you found the source of your illuminated bulb.

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Offline Lt.Del

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Re: brakes
« Reply #5 on: June 29, 2006, 02:38:00 AM »
Well, pulling up on the pedal did nothing.  

The light still comes on and goes off frequently.

I bled the front breaks...whew,, pulling those 31" tires on and off in 90 degree humid air, boy oh boy, what a chore....still, same thang..(southern lingo)

Now for Vilezombonie's info....

The light comes on when key first is turned on, then sometimes goes out, sometimes stays on.

The differential pressure sensor switch.....well, the switch may be bad, causing the light to turn and off.....well, as soon as I find out where it is, perhaps a replacement is in order.

Low fluid SENSOR...the sensor itself, due to knowing the fluid is definately good, well, as soon as the sensor can be found, i wanna replace that joker to rule it out.

Parking brake switch....doubt it.  If i push the parking brake down, the brake warning idiot light becomes brighter.   So, the switch seems to be o.k....atleast it knows when the pedal gets depressed by turing the idiot light bright.

Quote:
e) a short to ground after the bulb in the cluster ckt


hmmmm, this is the same circuit as the A/C. I know this because when I kept blowing a/c fuses, due to some short somewhere in the green wire going from cycle switch to compressor, the brake light automtically came on.  I ended up bypassing the green wire altogether by cutting it at the cycle switch, running a jumper wire straight from cycle switch to the compressor. So, i have no short anymore, the green wire is now dead from the cycle switch to compressor. It does sneak somewhere in the middle to the ECM--which i assume kicks the rpms up.  It never happened in the first place, so i dont care about rpms. But the 12 volts has been cut at the cycle switch. So this can't be it.

Quote:
f) an RWABS fault

I have no idea. RWABS???






SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

Edited by: SgtDel  at: 6/29/06 2:50 am

Offline roundedline

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Re: brakes
« Reply #6 on: June 29, 2006, 11:01:00 AM »

Offline Lt.Del

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Re: brakes
« Reply #7 on: June 29, 2006, 02:21:00 PM »
Thanks Chris.  I may be leaning more to the rear abs then.  Does anyone have a schematic of the control system of the ABS.  Is there a particular part that fails often in the subs rear abs? ---valve or electronic control?

I do recall once or twice getting a pulsating sensation when applying my brakes semi-hard--not hard enough to lock 'em up though, but that was a couple of weeks ago.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4


Offline Lt.Del

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Re: brakes
« Reply #8 on: June 29, 2006, 06:32:00 PM »
How would I know if the RWAL ECU (ABS control module) is bad?  

new abs module

Or is the speed sensor bad?


Or is the modulator valve bad?


Too many parts!!!  

If i take it to a shop, they are going to replace everything and charge top dollar for each part.  I'll probably drop $800.


UPDATE!!!...



Been doing some internet research on the RWAL, which GM used beginnng in '88.  It is an antilocking brake system for the rear wheels only, and only during 2 wheel drive operation (as compared to 4wd use).

raybestos has an online class on this.   Here it is.

A good overview of the RWAL is here Kelsey-Hayes

Here's an interesting ABS article





autozone site



This autozone site is pretty cool.  It shows component replacement procedures.

I think I may look into this myself.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

Edited by: SgtDel  at: 6/30/06 10:51 am

Offline Lt.Del

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Re: brakes
« Reply #9 on: June 30, 2006, 06:08:00 PM »
I believe this is my speed sensor.





SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

Edited by: SgtDel  at: 6/30/06 5:17 pm

Offline DnStClr

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Re: brakes
« Reply #10 on: July 03, 2006, 10:31:00 AM »
Sgt Dell, could it be a code that was saved by the ecm and just needs to be reset by the stealer, I mean dealer?  

Don St.Clair
Culleoka Tn

Don
87 Chevy Silverado

Offline Lt.Del

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Re: brakes
« Reply #11 on: July 04, 2006, 03:15:00 PM »
That is possible.  I really need to get the code read.  Won't it reset if I unhook the battery for about a minute....I thought that cleared codes.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4


Offline roundedline

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Re: brakes
« Reply #12 on: July 05, 2006, 11:51:00 AM »
Unhooking the battery for 30 seconds clears the code.  You can also pull the code with a jim clip stuck in the A/B terminals of the ALDL socket.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project