My 78 98 will build brake pressure in nuetral or park while running but as soon as I put it in drive or reverse there is nothing. You guys have any suggestions?

 

I suspect you are experincing a low vacuum issue with the brake booster. Use a vacuum gauge connected to a port on the intake manifold to verify there is sufficient vacuum when the transmission is in gear.

 

Thanks I will try that

 

Yes, low vacuum is the likely culprit.
-Check the line from the intake or carb to the booster.
-Verify the check valve (and its rubber grommet) in the booster is in good shape and allowing air flow in one direction only.
-The seal(boot) between the master cylinder and booster is known to fail after 40+years.
-Lastly, the booster bladder itself could be shot, Replacement or rebuild is two options if it's bad.