I have recently purchased a 1987 4Runner w/ 195k miles. The truck runs and drives great. One issue I have is that sometimes while the brake pedal is depressed the engine idle drops and revs back up repeatedly. As soon as I released the brake pedal the fluctuation stops. The problem does not always occur (never when engine is cold & only sometimes when engine is warmed up). Hoping someone can advise me on this...

 

Idle is too high causing the fuel cut to kick in from the brake system. Or the brake booster is causing a vacuum leak when the brakes are applied. To eliminate a bad booster, you can remove the vac line and plug it and try the brakes without the vacuum assist. Just be careful when you do as you loose your power brakes.

 

Thank you for the quick response. The idle does seem fast.