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78 base edition. Cruising along steady state brake warning light comes on. Stab a few times at next stop light and it goes off. Then randomly it will come on but will always reset by stabbing brakes. Bad combination valve? Other possibilities?
Parking brake switch. To determine which (prop or PB), pull the wire off the switch on the prop valve. Drive it and see if it still comes on. If it does, then it's the PB. If it doesn't then the prop valve switch is turning on, which may mean a brake issue. Another less likely issue is one of the 2 switch wires is being grounded some place other than the switch.
That light is a warning that your brake system is close to failure. (sometimes just a short)
You never said where the brake pedal position is when the light is activated.
Your jabbing of the pedal does indeed reset the proportioning valve. But you are also pumping-up the brake pressure.
First ! Pop the lid on the Master. What's going on in there?
Agreed, before assuming something is wrong with switches or wiring. Assume that the warning light is coming on to WARN you!
Pop the top off that master, are both chambers full?
Which one is low?
Follow that lead. Find the leak.
DON'T just assume your wiring is bad. It's your life on the line!
Thanks guys for your thoughts. The master cylinder is full. The light will randomly illuminate when I am not touching the pedal just steady cruise. Inside of tires is clean and dry. Will pull the old pads today to get a closer look at the pistons.
You could have a faulty Master and still activate the warning light. No leaks at the calipers. No drips anywhere. Master is full to the brim.
But the pedal is a little sluggish activating the light. How is this so?
The seals on the master cylinder piston allow fluid to flow past it. Could be just one seal or both front and back. You can not tell, just by looking at the rez.
Master is still full, but not a lot of pressure in the brake lines. Pedal drops some. Light comes on.
If you do suspect this scenario, I will tell you how to test the master isolating, at another time.
Thanks again for all the input. Yesterday I found some wetness on two of the four pistons on the left front caliper (both were inboard piston leaks). Therefore I have decided to overhaul all four calipers and install new rubber hoses while I’m there. Once complete I will report back with my findings.
Wise man says, go ahead and order a 55 gal drum of DOT3.
You will need it.
Also. Choose part suppliers wisely. Lots of reports about bad calipers from: such & such stores.
Hoses should be USA made.
You don't have to buy everything from one source.
Highly recommend a Motive Pressure Brake Bleeding Tank.
Sometimes you can find a used one on EBay. They will need a thorough rinsing with Acetone, new hose, but cheaper than new.
You can make your own MC lid adaptor for the pressure tank.
Click on my avatar and look for photo albums of brake bleeding.
Pulled the LF caliper today and broke it down. Lots of sludge in two if the bores but they were all sleeved with stainless. Cleaned everything up for inspection. Discovered i have two different diameters and lengths of piston springs. FSM makes no mention of this. See pic. Normal? Which spring goes where?
There has been lengthy discussion on whether the fronts are even supposed to have springs.
The rears do, to take up any slack between pads to the rotors.
Heads UP—if rears need them why not the fronts as well? Isnt the only difference in the fronts and rears the piston size?
Seems like the fronts get the fat springs and the rears get the skinny ones
Heads UP—if rears need them why not the fronts as well? Isnt the only difference in the fronts and rears the piston size?
Seems like the fronts get the fat springs and the rears get the skinny ones
All of the front and back brake calipers I've rebuilt (maybe a dozen) have springs behind the pistons – and they all appear the same length within the caliper. Here is a GM parts book listing for the front and rear springs (65/66 used a different piston):
Heads UP—if rears need them why not the fronts as well? Isnt the only difference in the fronts and rears the piston size?
Seems like the fronts get the fat springs and the rears get the skinny ones
I have no clue.
I don't rebuild calipers. I know I should. I buy rebuilts.
I did Google C3 front caliper springs and guess what.
This forum popped up with discussion (arguments) about it, whether or not the fronts get a spring.
It was about lateral forces in the front pushing on the pads when turning. Where as the rears are more stationary being on a axle.
Who knows?
Last edited by HeadsU.P.; Apr 2, 2024 at 05:30 PM.
With two different part numbers for front/rear springs i figure there is a difference. This also seems to match what I see on the Zip site. Guess i will breakdown a rear caliper and see what is there.
Before i proceed which spring was designed for the front caliper? Taller and narrow? Shorter and wider? Does not seem wise to mix springs in a caliber.
Brake warning light
