1985 Cadillac EL Dorado High Idle

My Caddie upon start will begin to idle slowly then rise up and up until the engine sounds like its ready to explode. The strange thing that stops this is when I press the brake the engine calms right down to normal operating idle and is fine. It currently has a "service engine soon" light on. Sometimes if I haven't started it in a while it will suddenly have a "service now" light on and nothing I do will slow down the idle, but after I pop off the ground on the battery for a little bit and fire it back up it will have no light on and acts just the same as if it had a "service now" light on except when I try to hit the brake to lower the idle, I hear a strange sound under the hood whenever I try to hit the brake. After the next start up it goes back to "service soon" and I'm back to square one. I've check the pcv valve and it is practically brand new. Any help is greatly appreciated.