[MB_70]

I had this problem ..... it was a combination of air in the lines and the wrong master cylinder. One of my symptoms was that we couldnt get any pressure when bleeding the system with a helper. I never had a reason to suspect this, but it was a $40 part that caused a $500 problem. If you bought the mastercylinder separate from the booster, it could be worthwhile to double check compatability through the vendors.

If that checks out, I would suggest bench bleeding the master cylinder, then gravity bleeding your brake system (if not done already). After gravity bleeding you will need to pressure bleed your system all over again with a helper.

You should also double check all of your lines to make sure they are not crushed, crimped, folded, and uniform in size. Also check the rubber lines to make sure they are not leaking.

Not trying to offend or belittle any skill sets or experience. Your post wasnt too detailed, and one thing we all have over looked (self included) is the fact that these are 40 yr old trucks that have usually been off road at one point in their lives. Mechanical skills vary person to person, as does integrity.

Its something small that is frustrating you right now. Best of luck, and let us know what it was ... might save someone else an afternoon of misery.