2nd Gen Non-Engine/Transmission Brake warning light.... again

[46mech]

I did a search of posts dealing with the brake warning light and read thru them all, but for the most part the gripes seemed to be that the warning light stayed on and the ABS light was on too.



I have different symptoms. Occasionally I will be driving along and the brake warning light will come on and a few seconds later it will fade away. I don't get the light during brake applications..... I had that going on a few months ago and found that my vacuum lines were weakened and collapsing. The only other thing I've noticed lately is that while doing slow speed maneuvering (more so while backing up) with light brake pedal application, I'm experiencing a mild "knocking" in the rear wheels..... I know it's coming from the rear brakes, not the differential.



Reading the manual, I understand the warning light is in circuit w/ the parking brake and the press. differential switch in the combination valve. I also see that the diesels have the low vacuum switch that is in the same circuit.



Since my fluid is fine, with no signs of leakage, and the vacuum lines from the vacuum pump to the power booster seem to be tight... ... ...



Do I have a vacuum leak in the power booster? Is that possible?

Do I have a bad low vacuum switch?

Do I have a bad combination valve?



Thanks

 

[46mech]

Another question though... .



I'm confused about the vacuum pump operation..... as the pump is operating, which way does the direction of flow go through the vacuum line between the pump and power booster? Is the pump pulling air from the power booster or pumping it to it?

 

[illflem]

The pump is pulling air from the booster.

Does your cruise work ok? It will also be effected by low vacuum.

Could also be that the switch on your parking brake pedal is set too close.

 

[46mech]

Honestly, I don't know if my cruise control is having any problems. I haven't used it in so long. By that, are you asking if it kicks itself off unexpectedly?



Thanks for the reply.

 

[illflem]

I'm not sure how low the vacuum has to go before it triggers the warning light but with low vacuum the cruise, your vent position control and 4wd will react slowly if at all. Might be hard for you to tell on the cruise if you rarely use it and don't have a baseline feeling for how fast it would react under normal vacuum.



My '95s will hold vacuum for several days without starting the engine. You can check this by popping off any of the vacuum hoses and seeing if they have leaked down rapidly after the truck is shutdown. An easy one to get at is the large hose going to the brake booster. It doesn't take much of a leak to bleed it down fast or get ahead of the vacuum pump output.

The most common place for a vacuum leak is at the cruise actuator under the driver's side battery due to acid from the battery rotting the hose.

 

[46mech]

Well, I went out and got me a vacuum pump/gauge. Going by the manual, I pulled the booster check valve out and checked to see if it was holding vacuum..... checked good.



Put the gauge in between the engine vacuum pump and the booster... ... The manual says I should get between 8. 5 and 25 inches depending on engine speed. I get a steady 29 and no difference when I raise the rpm's. I assume the gauge is working because when I applied the brake pedal the vacuum went down to about 20 inches.



I pulled the vacuum line off the low vacuum warning switch and put the vacuum gauge on it to functionally check it. Applying vacuum in accordance with the manual, the brake light went out when vacuum reached about 8. 5 inches..... as it should have. Based on that, the manual states there is a leak in the vacuum hose somewhere. Those hose was a little loose where it slips onto the switch so I cut off the worn out end of the hose and put it back together. I called it the day on this one and will keep ya posted as to whether that was the culprit.



In the mean time, I'm going to use my cruise control more to see if anything is going there. If I still get the light I'm going take a look at the cruise actuator, like you suggested.



Thanks

 

[GSykes]

ABS Light

Mine turned out to be the vacuum control valve located above the front wheel well on the drivers side. Both ABS & Brake light were on and ABS would fade every now an then. Replaced the valve and no problem.



1996 CTD Ext Cab Long-bed 2500 auto, 3. 54LS.

 

[46mech]

GSykes,

If the vacuum control valve you're talking about is the one I'm thinking of, that should be the low vacuum warning switch... ..... that's only only vacuum "anything" I could see on the driver's side wheel-well. Anyway, I was hoping it was as simple as that. The light has come on once since my last post.



I haven't really had the chance to check my cruise control operation like illflem suggested, as my drive to and from work is only about 10 miles of freeway drive and it isn't condusive to cruise usage. I think I'll take a closer at that area anyway..... I've heard others mention that they have had problems with battery acid getting onto the vacuum lines.



What still baffles me is the fact that my vacuum is reading 29 inches and the book gives a range of 8. 5 to 25.