How does the brake warning light work? (and many other brake problems)

[79Vette]

I was driving my 1009 the other day and the brake pedal got pretty squishy. Upon returning home I noticed the reservoir for the rear brake circuit with completely empty of fluid. I haven't found an obvious leak which is surprising, but there has to be a leak somewhere and I'm sure I will find it after I clean up everything and refill the system and run a few miles.

What surprised me is I didn't get the brake warning light on the dash. If I push the brake pedal all the way to the floor with the engine running the light comes on, but as soon as I let off the pedal even slightly the light goes out. I would have expected the brake warning to come on and stay on with the front circuit mostly empty of fluid but I'm not actually certain how the sensor in the proportioning valve works and would appreciate any insights you might have

 

[juanprado]

A master cylinder can leak internally and leak out of the rear. Check all hard lines especially if exposed to salt or deicing fluid plus rubber lines and wheel cylinders.

Parking brake has its own warning light circuit when engaged. the low fluid warning circuit has a sensor on the proportioning valve. 2 different ways to light the dash warning.

Double check the front brakes as the rear Port is not always the rear brakes.

 

[79Vette]

I refilled the fluid and bled the rear brakes and the pedal feel is back to normal. So that's good

The master cylinder could be leaking. The hydraulic lines coming off the brake booster have a lot of oily mud on them and I assumed it was the lines leaking. But it could be brake fluid leaking from the master down onto them. I'll clean up the area and that will make it much easier to identify any leaks.

Also I noticed both front calipers seem to be dragging pretty badly. With the wheel off I can turn the hub, but it takes both hands to do it. With the caliper removed the hubs spin freely as I would expect them to. Opening the bleeder on the caliper does not cause them to release, but removing the caliper and pressing the piston back into the board with a clamp works. As soon as the brakes are applied again the calipers lock up and do not release. Does this mean the calipers need to be replaced, or is there anything else in the system that could cause this? If it was a pinched or collapsed line I would expect opening the bleeder to release the caliper so it seems like the calipers are bad? I'm surprised both of them are sticking at the same time though...

 

[Jeepadict]

It's expected to have some residual pad drag on the front rotors when the pedal is released. You'll know a caliper is hanging or pad is hung up if you start getting the smell of brakes or clutch, and the hub area will be exceptionally hot to touch and possibly some discoloration on the rotor. If the front wheels (with tires installed) spin with minimal effort and give you a few revolutions before coming to rest on their own, I'd call that normal.

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[royalflush55]

If the front brake hoses are old they can collapse on the inside and not let calipers release also. They will let brake fluid go thru under pressure but not let it return.

 

[79Vette]

If the front brake hoses are old they can collapse on the inside and not let calipers release also. They will let brake fluid go thru under pressure but not let it return.

In this case, would you expect cracking the bleeder screw to release the caliper? That's how I've tested for collapsed hoses in the past and I assume it would work the same on a CUCV

 

[Jeepadict]

In this case, would you expect cracking the bleeder screw to release the caliper? That's how I've tested for collapsed hoses in the past and I assume it would work the same on a CUCV

Yep!

A CUCV is a mildly modified commercial off the shelf (COTS) unit. 97% of the truck is a common bone stock K30 pickup just as GM sold off the lot, then GM added blackout lights and a secondary/piggyback voltage system to make them NATO compatible.

The brakes on a CUCV are exactly identical to every K30 GM ever made in 1984!

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[Barrman]

Since they aren’t dragging due to pressure. I would suggest that you invest in some new calipers sometime soon.

 

[79Vette]

Well apparently I'm having all kinds of brake problems. Probably ought to change the title of the thread at this point...

I did replace the front calipers and master cylinder, since they seemed to be having problems. The pedal still feels terrible after running 2 quarts of brake fluid through it with a vacuum bleeder, so I'm assuming there must be air in there still. But I'm not sure what else to do, the vacuum bleeder usually works great on Dana 44/10 bolt calipers and is worked great on the rear drums last night. I guess I'll just bleed it some more and see what happens.

Then I decided to go through and check everything while I'm at it, and it turns out one of the rear drums is dragging slightly. Even with the adjuster loosened all the way there is about 1/4 of a revolution where the drum drags on the shoe. I couldn't hear it over the engine noise while driving but when spinning the wheel by hand it's obvious, and with the adjuster properly tightened it seems pretty bad
Swapping the drums side to side doesn't change anything, the passenger side still drags for about a quarter of a revolution regardless of what drum is on it. Can you think of anything that would cause that besides a bent axle shafts? I do drive the truck pretty hard and I guess a bent shaft is possible... I have 2 spare rear shafts but I really don't want to tear into the diff to pull the c clips just to find out there was something else wrong along.

There are no machined surfaces on the shaft to measure, but using a dual indicator on the edge of the hub face shows .015" runout on the good side and .045" on the side that's dragging. When I put on some random wheel spacers and measure off the machined surfaces on those show .039" runout on both sides. Since none of these surfaces are machined true to the axle centerline it seems like measuring and them is a waste of time and the results are inconclusive... I attached some photos of what I tried to measure, and how the shoes are installed if anyone sees anything wrong with that.

 

[Sharecropper]

If you can afford the cash I will recommend replacing those rear drums with a disc brake kit. That was the first thing I did on my M1028 in 2010, and it eliminated all of my brake issues. I also removed the rear brake proportioning valve per the GM Bulletin. My truck now stops on a dime. Just saying.........

 

[79Vette]

If you can afford the cash I will recommend replacing those rear drums with a disc brake kit. That was the first thing I did on my M1028 in 2010, and it eliminated all of my brake issues. I also removed the rear brake proportioning valve per the GM Bulletin. My truck now stops on a dime. Just saying.........

Did you remove the proportioning valve because of the disc brake swap? Or is that recommended for the factory brake configuration?

I've not heard of this GM bulletin, do you have any information I can use to look it up?

 

[Sharecropper]

Did you remove the proportioning valve because of the disc brake swap? Or is that recommended for the factory brake configuration?

I've not heard of this GM bulletin, do you have any information I can use to look it up?

I installed the rear disc brake kit first, but then later on I became aware that GM had issued Bulletin 88-320-5 in 1988 which provides instructions to remove the rear proportioning valve. I couldn't find my printed bulletin but you can read about it here - 88-320-5 Chevrolet Brake Control System TSB or TSB 88-T-151 (REAR BRAKE SENSITIVITY - HEIGHT SENSING VALVE REMOVAL) . After installing the rear disc brake kit, the truck braked and stopped great, however it felt as if the front discs were grabbing a little stronger than the rear. I made sure the rear proportioning valve was adjusted correctly, but then I discovered 88-320-5 and subsequently removed the valve entirely. After removing the valve, my truck braked and stopped like my sports car. I could not be more happy with the way the truck brakes and stops. If I remember correctly, somewhere in my rebuild thread I made reference to this.

Hope this helps.

 

[Barrman]

That height sensing valve is a must remove item in my opinion. It is only there because the trucks but mostly the Suburban’s couldn’t pass the obstacle avoidance lane change at high speeds. Slamming on the brakes while trying to steer caused the rear brakes to lock up and the vehicle to spin. So they fabbed up that leaky dangling linkage thing to basically turn off the rear brakes.

I have yet to be under a vehicle that has one of those installed to find it not leaking or at least wet at the leaking seals. They are a brake leak waiting to happen.

Removing it is also a great time to put a new flexible hose down to the rear axle.