Power brake issues

[Lattimer]

1970 C-10 that originally had manual brakes. The PO threw in a complete power brake setup from a donor truck, so I had a booster, brackets, braces, connecting rod to the pedal. I put it all in the truck, and ran the vac line to the unused vacuum port on the back of the manifold (engine is a 1968 327). Used the master cylinder that was already in place since it looks pretty new and doesn't seem to leak, and the manual brakes feel good. I also inspected all 4 brakes before trying to drive the truck and everything looks good. It has new pads on the front and the rear look to be about 75%.

Gave it a try in the garage and didn't feel any better than the manual brakes.

The booster looked like it had been through hell, so I assumed it was bad and got a new one from Advance. The master that had been on it was trashed and it probably had been leaking brake fluid for 30 years.

Installed the new booster, adjusted the rod for pedal height, and adjusted the brake light switch.

Finally got to drive the truck this weekend and it doesn't seem to work. Brakes still feel like manual brakes. Truck stops fine, no issues there, but it takes a lot of leg power to do it.

Anybody have any troubleshooting tips? I've never messed with anything this old until now, and swapping in the brake booster seemed like a foolproof mod. Too bad it isn't Lattimer proof.

[71K20chevy]

I suspect the vacuum line...maybe it's clogged or is collapsing?

[Lattimer]

Originally Posted by 71K20chevy

I suspect the vacuum line...maybe it's clogged or is collapsing?

Its new line. Just occurred to me that I should pull it off at the booster and see if its sucking.