I was sitting in a parking lot and had my foot on the brake while in D, I heard a pop/clunk and the brake pedal lost pressure at the same time. I also got a red Brake Light with ! indication on the bezel next to the RPMs that won't go out. At first I thought maybe the emergency brake handle wasn't all the way down. After I inspected...was pretty easy to follow the trail of brake fluid....I noticed fluid coming from the inside of the driver's side rear brake drum. Yes 91' Z28 with rear brake drums. I was going to upgrade the rear brakes but really just need to get it fixed for now. Assuming the rear cylinder seal popped as the brake line is still hooked up.

So what sensor detects that and triggers the brake light...if it a pressure sensor of some sort on the brake master cylinder. Just curious.

 

It's in the proportioning valve, right below the MC. It's normally "centered", and moves one way or the other when one end of the car has pressure and one doesn't.

Once you fix the problem it will reset itself by standing on the brakes hard and letting the equal pressure in the front and rear sections push it back to the center.

 

You are very fortunate it didn't happen on the road. Lucky day!

 

Funny. When that happened to me it was a front brake line while I was just pulling onto the interstate offramp, running late to work.

 

Nice thank you!

Oh but it did I was 9 miles from home going down hill lol. When I got home and went to park is when I noticed the trail of brake fluid.

It's definitely an unnerving feeling.

 

No doubt...

My 2004 Avalanche did that not too long ago while I was pulling a trailer, on a SUPER busy road, coming up to a traffic light. Except in my case it was a steel line that had rusted through and decided to pop right then. I was lucky; managed to stop before I hit the cars sitting at the light. Put a stainless hard line set on it.

First car I ever had was a 60 Chevy. I lived in Memphis at the time, worked not far from Graceland. Any of you that have ever been there might remember that if you come off the freeway on EP Blvd and go south, you go up a hill, then back down... as I was going down, coming to the light, a wheel cyl did that. But being a single-cylinder system, that took out THE WHOLE brake system. I was helpless. I rear-ended a Pinto, and it rear-ended ANOTHER Pinto. And dammit, neither one of them exploded. I had owned the car for like 2 weeks, and that wreck cost me around $1200 in early 70s (before stagflation) money. My butt hurt for YEARS.

Nowadays at all times I pay CAREFUL attention to the operating condition of my brakes.

 

That's a great story...funny about the Pintos but lucky for you they didn't explode.

 

It's ok. I'm sure you'll do better next time.

 

So it looks like the worst design on a drum brake kit goes to the engineer or production manager who decided to use 70's drum brake technology on the 91' during the rear disk brake shortage. I found that the retaining clip the holds the brake wheel cylinder on the back popped off and let the wheel cylinder basically pop out of place, the entire seal and piston popped out of the cylinder. I purchased a new wheel cylinder that came with the new retaining clip. The two bolt design to secure the wheel cylinder should have least been incorporated. So now I have to hold the wheel cylinder in place while I use a socket to pop the retaining clip over the wheel cylinder from the back to install. Not hard but would have been a lot more convenient if GM would have just used the two bolt design.

 

Having trouble getting the dash light to reset? I have stood on the brakes pretty damn hard. Is it possible the proportioning valve got stuck all the way back or forward. I bled the brakes and there are no air bubbles that I can tell in the system. Damn light. I disconnected the connector on the proportioning valve just to look at it lol. It's a different type connection for sure.

 

Yep, they'll do that. You can unscrew the yellow switch from the top of the prop valve, and look down in there... There's a rod that runs from front to back, with a V groove in the middle. When there's an imbalance front/rear the rod will slide forward or back, which moves the V groove off center, which forces the pin on the switch up and turns on the light. What happens is all the sediment and trash in the bottom of the brake reservoir will collect in the prop valve and when the thing finally moves after 25 years of living in the same spot, it'll get stuck. The trick is you can get in there with a scratch awl or something similar and push it back to center and as long as you don't have the imbalance anymore, the light will stay off.

 

Trying it tomorrow. Should I relieve the pressure from where the prop valve spring is first (at the front)? I saw the yellow cap under the connector if I pop it off is brake fluid going to squire up out of it?

 

No need to relieve pressure, it's not in the fluid circuit.

 

I couldn't get that thing to budge. Appreciate the recommendation though. Can the prop valve be disassembled or do I need to get a new one? I backed off the yellow fitting with the connector hooked up and the spring loaded pintle disengaged so the light is out on the dash for now. I didn't want it to burn out the bulb until I could fix it.

 

You don't need to replace the valve, you could cut the tip off the switch and screw it back in, and you'd never know the switch was disabled. Do you really need a light on the dash to tell you your brake fluid is low or something in the system just burst? Usually the complete loss of braking is more obvious than the bright red light.

You could try removing the valve and doing it on the bench, but you'd have to bleed the entire system, which is likely to force the switch back off center in the process. Or yeah, you probably need a new/used prop valve. Just keep in mind if a car is getting parted, when was the last time it had it's brake fluid flushed? No one ever flushes the brake fluid, but it's got a change interval something like once a year.

Oh yeah, to the best of my knowledge that part of the valve can't be disassembled without destroying the valve. I've got mine that got stuck around here somewhere in a junk box. I've looked at it quite a bit, never saw a way to get the valve far enough apart to remove that piece. Probably possible if a person knew what to do, or didn't mind destroying it.

 

I could do that but I'm not a hack when it comes to fixing my cars. I just as soon repair it correctly, even when it has come to fuel pump replacements I don't cut holes in the cars I drop the tank. The car I have looks new for a 29 year old car. I want to keep it that way with all of the parts working correctly so I have disconnected the connector to eliminate the light from the dash.

The brake fluid did look trashed I suctioned out the brake fluid with a turkey baster in the reservoir and it was just black and muddy, I cleaned out the reservoir and added some new fluid. So yeah I agree when I replace the brakes the entire system needs to be flushed out. I will probably end up buying a new proportioning valve as I can't get this one unstuck. I tried to move it with the scratch awl as suggested and it will not budge. That is why I was wondering if I took out the valve spring if the piston would move more easily.

Thanks for helping out with the info though. Everything was working good until that clip on the back of the wheel cylinder broke. What a poor design to use for mounting the wheel cylinder. they must have really been desperate in 91' to use 70's drum brakes on these cars.