Looking at a 78 Ford LN800 with the 429 and auto trans. Seller says it is having brake issues and thinks it is the vacuum pump. Has air assist brakes. Brakes work at lower RPM but at speed does not maintain pressure. Thought the vacuum came from the engine and not a pump but then I am not yet up on this sort of rig. Will likely be looking at a diesel swap and perhaps a manual tranny later but for now have to get it home. Any insight would be greatly appreciated. Thanks.