1990 S10 brake light stays on

I have a 1990 S10 Blazer in which the brake light on the dash stays on. It doesn't have a 4WAL EHCU valve and is only 2 wheel drive. I have reset the computer unit beside the master cylinder, the light goes out for a while but comes back. I removed the connector from the combination valve and shorted it to ground and the light comes on, when short is removed it goes back out. When I put the parking brake on it gets brighter. I have tried bleeding both, front and rear brakes.

Any suggestions?

Are the brakes working right? Either the pressure differential switch is bad, or there is a difference in pressure front to rear. Try pumping up the brakes. If you can get the light to go off, it is probably the master cylinder seals or the wheel cylinders. It could be anything brake related, but I've seen more wheel cylinders and master cylinders than anything else.

fill the system up with fluid first then if it stays on get a friend to step on the brakes while you crack a front bleeder.

This trick will often center the pressure differential switch and cure the problem