Have had the 74 bus for 15 years. Recently noticed a lot of backfiring, and my brakes were fading. Basically needed to replace the master cylinder, (another story) and spent some time checking hoses and the One Way valve near the firewall. Those old hard plastic hoses have gotta go. I disconnected the vaccuum hose at the booster, and blew down the hose, (clean it first). You should have airflow in this direction. Then carefully apply vaccuum to the line. It can taste a bit yuck. If you have any leaks you can hear it. Basically you should not have any pressure loss at all. I had air going everywhere. So down to the hydraulics shop for 2 metres of 10mm air line, and a quick visit to our mechanic for a new one way valve (ours was not). This is a simple job, you remove your old hard non-sealing hoses, measure them against your flash new hose, add an extra 15-30mmfor good measure, cut and refit with new hose clamps. My brakes are excellent, and the backfiring has stopped, now just a low grumble during decelleration.