78 F-150 Braking problem

[hockey11bart]

I have a 1978 Ford F-150 with a 351W engine. Taller Cam, 4 barrel carb with a high rise intake, headers, and no heat or ac unit. The truck comes to a stop just fine but once completely stoped the truck RMPs increase from 1000 to upwards of 2000-2500. The truck will lurch forward and loose all power breaks. I almost have to stand on the brake pedal to get it to stop and the engine continues to rev realy high. As soon as I let off the brake the truck lurches forward a few feet and the RMPs immediately drop back to normal and the truck will drive fine. The problem does not always happen but it seems to happe whenever I have to break really hard or the engine is runing really hot. I assume it is a vacum problem. I tried re-runnng the vacum line to the booster but that didn't work. There is no EGR valve due to the headers being installed so I can't check any vacum problems there. I have been told that it could be the vacum advance on the distributor but that wouldn't make the power breaks not work. I think it might be because the engine is got so many modifications that it is hard for it to produce enough vacum. Do I need to find a leak or do I need to purchase an electrical vacum assist pump to help it produce enough to power the breaks at all times?

[toivo]

check your throttle position sensor and make sure it doesn't have a bad spot in it. toivo