I've had a strange thing happen in the last week. When the temps have dropped into the 30s overnight, when I start my 95 Impreza (133k), the brake light on the dash is on. Emergency brake is released. After driving a few miles, the light goes out. The brake fluid reservoir is full and the brakes work fine. Had a fluid flush last year after having brakes replaced. It only happens in the morning when it's been cold overnight, doesn't come on in the afternoon for the drive home. Any thoughts? Sensor sticking til it warms up?

Brake light only comes on for 3 reasons....

 

1) warning light "test" with key on......out when charge light goes out.

 

2) Park Brake handle up......switch is on the handle.......Perhaps spilled, sticky gunk, gumming the switch up espescially when cold?

 

3) Fluid level in resevior......Switch in the resevior.....Sometimes can be stuck down when not quite full......Top it off a bit?

#3 gets my vote.

#3 gets my vote as well. Things (including brake fluid) contract when cold and expand when hot. The cold fluid has dropped in volume overnight, arming the low fluid brake light. Once it warms up, the light goes off.

Obviously the brake fluid level has dropped, due to brake pad wea,r where this light on then off situation will continue until the brake fluid level is topped off again.

is there a MC switch in that circuit too? something that triggers the light if one hydraulic circuit's pressure is different from the other?

is there a MC switch in that circuit too? something that triggers the light if one hydraulic circuit's pressure is different from the other?

 

Nope.

If the level is correct, you have moisture freezing in the vacuum line.

Remove hose run a little gas through it and let it dry thoroughly before attaching.

 

O.

Thanks for the responses. I'll check the fluid level when it's cold in the morning and see how it is. The light was on this afternoon leaving work, but only for maybe a mile, then it went out and stayed out for the remainder of my 25 mile commute.

It could be that your brake fluid is saturated with water since it is very hygroscopic. Otherwise #3