Posted by Ray on April 17, 1999 at 20:45:42:
In Reply to: Brakes Fix ?? response 4/06 and 4/11 posted by STP on April 17, 1999 at 19:43:21:
I think you can forget about the wheel cyl. being the problem. I had the same grimlin you do and I have 4 wheel disc brakes. If you look in the filler opening of your master cyl. you will see a small hole approx 1/32" dia. when the piston moves forward it covers this hole and stops the fluid from venting back into the resevoir allowing the piston to build up presure. When you release the pedel the piston moves back past the hole allowing the built up pressure to vent back into the resevoir. This little hole is also there to allow for thermal expansion of the fluid. When you are driving the brakes heat up, heating the fluid causing it to expand. This is your problem this hole is being blocked either by the piston (due to misadjustment of the push rod or you have contamination (if you have old hoses it could be bits of rubber from them floating around in the fluid) I solved my problem when I replaced my master cyl. (my brake lights would come on when the truck sat in the sun and the fluid heated up!)
Your problem is probably thermal expansion.