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I replaced all my calipers and flushed and bled the system twice (once myself and once at a race shop). Brakes seem to work fine and no leaks. However, my brake warning light on the dash (74 model) won't turn off. Any suggestions as to where else I should look or is this just a bad switch?

Thank You,
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Is it the shuttle valve? It senses a differential in pressure between the two (F/R) braking circuits to warn of failure. You may have to re-center it by cracking a front or rear bleeder and pressing on the brake pedal which will push it back to center and turn off the light.

Or, possibly one of your old brake pad wear warning light wires is grounded?
Nope, not grounds and not the switch. It is varnish in the shuttle valve cylinder.

When you open bleeders during bleeding you create off-balance pressure in the system. Thus, you have one end still blocked, with full pressure, and one end open. This is the pressure differential the shuttle valve was meant to detect. But after years of no movement, varnish will build up the the cylinder for the shuttle valve, it moves, and it is stuck in place by the varnish.

Hit the brake pedal HARD several times. No need to be moving. Just turn on the key so the light is on, and go for it. Hopefully, the valve will move back to center and allow the plunger on the switch to extend, and open the light's circuit.

If that doesn't work, hit it again HARDER.

If all this fails, the next course it to remove the shuttle, clean, and put it back.

Do a find search for 'shuttle' and you'll find many posts on this.

The switch in question here is the one on the right. The other switch is the brake light switch.


NOTE: If the light is still on, then the shuttle valve has moved to one side, and the flow of fluid to that side (end of car) is somewhat restricted. This can affect your braking efficiency.


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Seems to me that if differential pressure moved the shuttle and the switch made contact, opposite differential pressure will move it back, not just jamming the pedal as hard as you can. Where's the differential then?

If it's gummed up with varnish and stuck, that's another thing. One last check would be to pull off the wire. The light should go out and you'll know you're on the right track anyway.

I looked at my '74 wiring diagram and it's clear that a different light is lit by the pad wear sensors. However, your E-brake warning light is the same light as the one illuminated by the differential switch, are you sure your handle is all the way down?
not just jamming the pedal as hard as you can. Where's the differential then?

Correct you are, on paper it shouldn't really work. But all I know first hand (and others before and after me) is a HARD pedal jam will often set things right.

If you could determine which way the shuttle piston moved to be stuck, you could open a bleeder valve on the other side and hopefully the pressure differential factor would then force the piston away from being stuck...but perhaps it would just now be stuck on the other side. Wink

pull off the wire.

Good check, too.

Last edited by lf-tp2511
..... and if Larrys brutal-stomp still doesn't work, its possible to pull one brake line off the valve and shoot a SMALL amount of high pressure air in one side from a compressor. This WILL move the shuttle. It helps if the shuttle valve light switch is out; the shuttle o-rings if in good condition will keep most or all the brake fluid from leaking. Obviously, this also does nothing for removing the crud that caused the problem.
To figure out which direction the shuttle is stuck in so you know which line to disconnect, pull the light switch out and use a mirror to figure out where the shuttle is located, referring to Larry's exploded view.
All this is why my valve has been on a shelf in the shop for 20 years. I reaaly don't need an idiot light to tell me to check my brake fluid levels and pad thicknesses....
On other cars, the movable shuttle has return springs on both ends to sort of help it recenter. In our valve, all we have is the slope machined on the shuttle and the spring in the light switch. If varnish etc gets in there, that little spring's not enough to recenter the shuttle.
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