Look at the bottom of the master cylinder where it is bolted to the brake booster. Do you see any fluid or paint peeling away from the booster? Is there any fluid droplets on the inside of the vehicle where the brake pedal rod goes through the firewall into the brake booster?
Remove the master cylinder mounting bolts (remove the vacuum supply hose from the brake booster first) pull the master cylinder away enough to look inside the booster for brake fluid leakage and the back side of the master cylinder for fluid leakage. Is there any? A leaking (leaking vacuum) brake booster due to age or brake fluid leakage can cause a large vacuum leak causing the engine not to run smoothly or at all depending on to severity of the leak. Try this and get back to me, TY
Monday, September 9th, 2013 AT 9:22 PM