crd86vette

I am in the process bring 78 Corvette back to life. I have replace all rubber lines / master cylinder/all cailpers,,,,,,,now I can't get the brake warning light on the dash to turn-off. I have good brake line pressure on all for wheels.I have a good firm pedal. Has anyone had this problem?

Jud Chapin

Double check the brake fluid level. If it is a little low, it can trigger the light.

crd86vette

Thanks for the quick response. The Master cylinder is full. Does anyone know if is possible to replace the warning light switch on the proportionate valve?

Jud Chapin

If the brake light wasn't on before your work, I suspect something other than the light is the problem. Have u driven the car? It should stop on a dime if everything is in order with no sponginess in the pedal. Bleeding can be a tough job on these cars sometimes, especially the MC. Did u bench bleed it?.

crd86vette

I only drove the car onto trailer when bought it. So Honestly I don't know if the light was on before. The car has set for 10 years before I bought it. A mouse had chewed some wires and vacuum hoses so I had the dash out of the car when I started working on the brakes.

After repairing the mouse damage. I started on the brakes. When I took the tires off I notice that rear calipers were wet so I replace those. then I drove it about a mile and the right front brake was dragging. Since the car had be sitting I decided to replace everything. Hoses, Master cylinder and all calipers.

I did bench bleed MC. When the light would not go out I unplugged the proportionate valve switch to ensure the parking braking wasn't causing the light to come on. The light goes off with the switch unpluged. So my thoughts turned to the proportionate valve being stuck. But as I understand if it is stuck you have restricted flow to either the front or brakes. which I really good flow at all calipers. So I thought maybe the warning light switch so bad.

Its seems to have a firm pedal. But I haven't driven the car since the light is on. I think I will try driving it. It just makes me nervous seeing that light on.

Jud Chapin

Try the brakes as soon as the car moves. The car should stop dead immediately and u should have a firm brake pedal. Don't go far and be very careful until u r sure the brakes are properly working.

crd86vette

Well after driving the vette it seems the rear brakes aren't holding. Which is strange because seems to have good line pressure when I bleed them. So I guess the proportionate valve is stuck.

Has anyone had luck unsticking the valve? I have tried bleeding one front and rear caliper at the same time as some have suggested on this site with no luck. Some say to blow dry compressed air in the line. Is this done at the rear caliper feed line? How much pressure should I use?

AboveTheLogic

Give the pedal a few nice hard kicks and see if the light turns off.

gcusmano74

You may have shoved the switch over to one side when you bled the brakes. It is designed to light the brake warning light when a leaking line/hose/caliper creates a pressure imbalance between the front and rear system.
Try applying pedal pressure to the brakes while an assistant cracks open a front brake line. Then do the same thing with a rear line. That may center it. You may have to do this a few times.
Good luck.

crd86vette

After few more attempts to bled the system the way you suggested the brake pedal felt better but the dang is still on. So I once again drove the car down my gravel driveway. When I stop quickly only the front brakes are holding. So once again my thoughts turns to the proportionate valve being stuck. I decide to take my hand pump and push clean brake fluid through both rear bleeders valves. This fills the master cylinder. So I try the front... same thing... does this means the proportionate valve is centered? If so I guess I need to just keep bleeding the brakes. I have used 3 bottles of brake fluid so far.

How do you guys bleed these C3? What is the secret? I have read it can be a pain and I believe it. In my 30 years of turning wrenches I have never had this much trouble bleeding a GM brake system!!

Jud Chapin

[QUOTE=crd86vette;

How do you guys bleed these C3? What is the secret? I have read it can be a pain and I believe it. In my 30 years of turning wrenches I have never had this much trouble bleeding a GM brake system!! [/QUOTE]

I've always gravity bled mine (many Vettes over 35 years) and never had a problem. It's slow but works.

crd86vette

I think that is my next step. I am going out of town so It be will be next weekend before I can try. Can you explain what steps you take to gravity bleed ? https://www.corvetteforum.com/forums...ilies/ack2.gif

Peterbuilt

Above is the best way. Apply all the pedal pressure you can muster and open a front bleeder.

You are bleeding both rear (inner then outer) bleeders, yes?

crd86vette

Hi petes74ttop,

Yes I am starting with the right inner then outer, then to left inner then outer. Once finished with the rear I move to right front then left front.

Jud Chapin

To gravity bleed, start with the caliper furthest from the M/C which would be the RR and work forward. The rears have 20 bleeder screws so do both. Just open the bleeder screws, I do one at a time, using a clear plastic hose connected to the bleeder and let them drip in a 2 cup measuring cup. I usually just bleed until the clean fluid appears, but in your case, all your fluid s/b fresh so I'd probably take at least a cup from each bleeder. Naturally, keep your eye on the m/c fluid level as you don't want to empty it or you'll be back to square one. This process can take a couple of hours.

Speaking of the m/c, you said u bench bled it. Be sure u got all the air out in doing so.

crd86vette

I think I will try your method next week. Maybe after 5 days I will have a clearer head. I THINK I got all of the air out of the M/C. I will start with the M/C just to make sure.

AboveTheLogic

I had the same issue with the light after bleeding brakes. Kicked the pedal and the light shut off. It came on again a few more times, requiring a quick kick to the pedal to shut it off, and eventually stopped coming on.

mac79vette

I have never had this problem on my vette but every time I have to bleed the brakes on my 71 c10 I have this happen. To fix it I have to open the lines slightly at the valve while some one apples pressure to the brake pedal. Always works on the truck, might work on a vette but it's harder to get to the valve on the vette.

thegazman

On my 79 the valve was stuck and would not reset. I had to take the proportioning/distribution valve off and clean the 35 years of gunk out of it.

gcusmano74

Gravity bleed C3:
Put six or eight sheets of shiny magazine paper under each front tire. This allows you to steer the front wheels each way without much effort.
Jack up the REAR of the car only, and remove both rear wheels.
NOW crack open the bleeder screws, one at a time, rears first. To reach the front calipers, just turn the wheels back and forth and bleed as before. Not a bad idea to TAP (just tap) the calipers with a block of wood or a rubber mallet when the screws are open, to help dislodge any bubbles.