captainricks

Brake warning light is on my 1989 C4. The master cylinder was replaced, bled and new fluid. I understand it could be a faulty master cylinder warning switch and\or parking brake switch. How can I tell which one? It seems the master cylinder warning switch is easiest to replace, but would the light go out if the electrical connection is disconnected. Help! light is annoying. P.S there is normal pedal pressure. Capt. Rick

jdog0411

Have you checked your brake pads? I believe there is a wear sensor that will illuminate the light when the pads need to be replaced.

captainricks

Yes brand new

Hooked on Vettes

When the parking brake is on a switch is closed which places a ground on
the Brake indicator bulb.

The Brake Master cylinder also has a switch which grounds the bulb if
the brake fluid is low or there is a pressure problem.

So you need to disconnect each switch plug to determine which one
is causing the ground.

captainricks

I am not a mechanic so if I understand. The master cylinder switch is on the master cylinder correct. If I disconnect the electrical wire to it and it is the source of the warning light being on, the dash light should go off. Right? If it stays on I then need to look at the park brake switch. Where is the park brake switch on and 89 C4?

Craftman555

(try this at your own risk......)
Yes, the "white plastic switch on the master cylinder is there to indicate that there is a pressure differential problem between your front and rear brakes. If there is a leak in one or the other (front or rear), there is a little shuttle that will slide from center (open) position to a side or (closed or grounded) position. If you remove the singe wire that goes to this switch and the light goes out, this switch is the culprit. There are a couple of possibilities. First, there may actually be a problem with your brakes causing a pressure difference in the system. Thoroughly check for leaks (do you ever add fluid?). Second, after these remain stationary for a length of time, if for some reason (bleeding the brakes) they are shifted one way or the other, they will stick and not recenter. I had the same thing just happen to my 89. If you remove the switch, (be careful, there is a spring with a pin in the bottom of the switch) look inside and you will see the shuttle. There is a groove in the center of the shuttle that should be in the center of the open hole were the switch was. I used a precision mini torx driver to carefully shift if back to center, re installed the switch and voila....good as new.
Just a couple of notes:
1)There shouldn't be any fluid inside where the switch is....it's sealed.
2)It did take a fair amount of force to unstick mine, just be very careful not to damage the threads or the shuttle.

Good Luck!

Hooked on Vettes

That's correct. The red arrow points to the parking brake
switch connector. Back side of the lever mechanism.

captainricks

Thanks for the info greatly appreciated. Ill see if I can figure it out this weekend. Capt. Rick

captainricks

I disconnected the park brake wire and the master cylinder pressure switch wire light is out. After trying different combinations I narrowed it to the pressure switch. I'll order the part and replace it.

Hooked on Vettes

Did the light come on after you replaced the brake master cylinder?

Craftman555

If your sure the master cylinder pressure switch is setting the light, replacing the switch will not solve your problem. After installing your new master cylinder and while bleeding the brakes, the inner shuttle interpreted the brake bleeding as a brake failure and did exactly what it's supposed to do by moving to the low pressure side to stop the loss of fluid and setting the light. This is a common problem while pedal pump bleeding the brakes. You can try to sharply jab the brake pedal to recenter the shuttle but I have never had any luck without creating a loss in pressure by opening a line on the opposite side of the system and then sharply jabbing the pedal. Unfortunately this also necessitates bleeding the brakes again to remove any possible air let in and the potential to set the light again...... This is why I invested in a pressure bleeder.
You could also try to recenter the shuttle by my above method.
Good luck.

captainricks

I am being to understand. The work was done by a local shop. Supposedly the mechanic is a master well versed in brakes. I had a tough time trying to explain to him to check the parking brake switch to make sure it was the master cylinder switch. He was the one who said to replace the switch which has now been ordered at $50.00 for part only. So now when he installs it and the light stays on. I am wondering what he will say? Probably live with the light on. Also I am wondering if the master cylinder is faulty. He put in a rebuilt unit. Hopefully I will show him your reply and he will follow through. Does bench bleeding the master. Cylinder prevent the shuttle from being off center when installed? Like I said I am not a mechanic but I think I had better knowledge than mechanic. One more question with the shuttle off center am I only using one half of the brake system?

captainricks

One more question is the shuttle on the switch or is it inside the master cylinder? I had the mechanic give me the old one and the hole is so small where the switch screws in that I don't think I can do the repair you suggested myself. Thanks Capt Rick

Craftman555

The shuttle is inside the master cylinder. You can remove the switch without any fluid escaping and see. Just be careful...when you remove the switch, there is a spring with a nail like pin that is on the end of it that will fall off (trust me I know first hand). The shuttle is like an hour glass on its side. There will be a small grove in the center that you should be able to catch with a very small tool like a precision screw driver or a tiny allen wrench.
If you had a mechanic do the swap and he gave it back to you with the light on, make him fix it.....

JrRifleCoach

Tell your "master mechanic" to stuff that new switch up his arss and recenter the shuttle valve!
You'll need to remove the big hex plug at the front of the MC and push or pull the shuttle valve until centered.

IF, fluid is leaking out the white switch body, the shuttle seals are leaking and need to be replaced.
Seal kits for the MC are still available through Rock Auto and others for around 60 bucks.
Don't let him rip you off for a new/rebuilt MC

The shuttle valve is shown with the switch tip in the groove for normal operation.



My shuttle valve seals were leaking causing the warning lite to illuminate.
Plus the fluid leaking out the switch body was a clue.



.

captainricks

Thanks again. Very helpfull Very frustrating dealing with mechanics.

Hooked on Vettes

PRESSURE DIFFERENTIAL SWITCH
The pressure differential switch is connected to the
brake warning lamp. The switch is triggered by
movement of the switch valve. The purpose of the
switch is to monitor fluid pressure in the separate
front/rear brake hydraulic circuits.
A decrease or loss of fluid pressure in either
hydraulic circuit will cause the switch valve to shut-
tle forward or rearward in response to the pressure
differential. Movement of the switch valve will push
the switch plunger upward. This closes the switch
internal contacts completing the electrical circuit to
the warning lamp. The switch valve may remain in
an actuated position until repair restores system
pressures to normal levels.

Either the brake master cylinder has a problem or the fluid pressure
between the front and rear brake system is not correct.
Correcting the problem should correct the brake master cylinder shuttle
position automatically.

JrRifleCoach

Key word is should. Reality is the seals are not very supple in a twenty plus year old car.

captainricks

After three hours at the mechanic who insists it is the switch. Went home used a small eyeglass screw driver and could see the spot you referred to. Moved it over and voila. light out I am thrilled. Thanks everyone who helped. Capt. Rick

Da Mail Man

Greetings All,

1988 corvette coup......I have the same/similar problem, UGH!

History:
1)...I had a master cylinder rear seal leak (dual reservoir) and replaced the MC with a new one.

2)...Then, to add insult to injury, found that after I bled the mc and started to pump pedal after lines connected, found pedal rocketing to the floor. ...I looked behind drivers rear wheel and found a major brake fluid leak.

3)...Examining the area, found that a hard brake line had ruptured with a pin-hole leak in same....(This is an ABS equipped car).

4)...After waiting for ordered parts, buying flaring tools, etc, and a day w/out rain, replaced the defective portion of brake line, bled all brakes, abs, etc..brake pads ok.

5)...I took it out for an "around the block" slow drive and found that my brakes were as $hitty as usual but, that there were no leaks....No dash warning lights were on either.

6)...I "THINK" I may have worked the parking brake (L of drivers seat of course) and did see the dash brake light come on and off as it is supposed to. Before I exited car, placed it in it's normal "rest" position....The common denominator is me working the brake lever EXCEPT, that the brake light went "off" before I shut down car (I suppose something could have been sticking preventing the light from coming on before I quit it...

7)...The other day I started the car and found the dash brake light "ON" (no abs light - just brake warning light).

8)...Having had this problem many years ago, I began to "work" the brake handle as I did then, hoping to kill the light, but, to no avail.......Having an AutoX-ray scanner, I pulled codes and no current or stored codes (I did this because it was simply time for me to do so).......

9)...I did notice that while "gazing" at the dash light, it got a TINY bit brighter when I pulled the handle, leaving me to believe that something may be shorted under the parking brake cover to ground..... I say "ground" because I am sure the resistance for the connection(s) are lower going through the switch with presumably copper contacts for low resistance then would be going through metal, possibly painted metal, having a higher resistance......Will attempt to remove cover w/in next few days (60% chance of rain all week)....

Briefly, how does/do I remove the cover over the parking brake?...I simply don't recall at this time.....Thanks.

My thread on the brake line replacement with picts: