1994 Ford F-250 7.3l Diesel - Help diagnosing Brake Power assist issues

[mobiledj911]

Help diagnosing Brake Power assist / Braking issues
The power brakes on my 1994 F-250 recently began to work erratically.
By the above, I mean that sometimes when I hit the brakes they work fine and other times I have to push very hard to get the brakes to work (no power brakes, manual effort).
I have a few questions:
1. What is the procedure to diagnose these types of issues?

2. If the Power Assist module is bad and needs to be swapped out, (unit on the firewall with master cylinder mounted to it), does this require bleeding the brakes or no?
3. If the Power Assist, should I buy the one with or without the master cylinder on it as they are close in price and would make that part of the braking system new.
4. Have done brake work, but never had to bleed them, any tips?

[drinkypoo]

Test the vacuum pump. If you want the precise test procedure I can probably find it; you can probably find it on here with search. But it should make pretty deep vac at the tree connector that it feeds.