I have a 390 with C6 in my 1968 Ford Ranger. If it's cold or slightly warmed up it'll melt the tires with or without power breaking it. After you drive it a while and it gets hot then try to spin again it bogs down and won't rev or spin. Even power breaking it in 1st gear it won't do anything but load up and sit there. My first thought was trans issues but now I'm not so sure. Any ideas? Here are some specs just in case it's needed.

390, Street master intake, stock heads ported and polished with larger valves, 10.1 compression pistons, Headman ceramic headers, Edelbrock 750 carb, MSD distributor and 6AL ignition box. C6 was rebuilt front to back with shift kit etc and 3000 stall converter.

 

Vacum leak, the choke is compensating for it till warmed up.

 

It produces very little vacuum due to the high duration cam. Brakes stink due to it too. : ( It just recently started though is what is weird about it. Didn't do it when it was first built. I have had terrible luck getting this thing tuned and been through 3 Holley carbs and 2 Edelbrocks. It's a Comp 292h cam if that is needed. I'm lost. Can't find any vac leaks so far, but still going to check some more things.

 

A 292 advertised duration cam shouldn't be that bad for vacuum. Yes it will have less than the stock cam which should read approx 15-18 inches at idle (with everything right), but not much less if everything is set correctly and you have no vacuum leaks. Going by your comments about the brakes now being a problem I'm assuming you have power brakes. The booster for the master cylinder would need around 12 inches of vacuum to work properly. Any chance you have a damaged diaphragm in your booster, or the vacuum hose going to the booster being bad? With the truck warmed up, how does the truck run with the brake booster disconnected from the intake and that port plugged?

 

Last time I checked vac it was only around 8 on my vacuum gauge. It wasn't power brakes factory it's all been added on new. If you pull the vac hose out of the booster you can here the air rush in with vehicle off so i'd think the vacuum there wasn't leaking. I'll try your idea and unhook it and plug the intake port and give it a spin.