I noticed RPM drops yesterday when in the car park and I had my foot on the brake. I then proceeded to pump the brakes to tsst it out and the car drops to 450rpm and almost stalls. If I stop pumping it comes good in about 5-7 seconds. I'm thinking vacuum leak as I plugged the vacuum hose at the booster side and had no issues. Is it likely the booster itself needs to be replaced or is there something I can do further to test?