Two questions...
Do these usually go bad?
The bike I purchased has been sitting for a little.
Want to know if I should replace this or just clean it before putting the bike back together.

Question 2, on the bottom of the fuel pressure regulator there is a black vaccum line
Part # 11 72 7 545 323

It has become extremely hard and is cracking
Can I replace it with any other vaccum line of the same size?

Thank you in advance

I replaced mine when I brought the bike back to life . Do it all and then ride with confidence that it will get you where you're going and back home always . Only fix what you feel is bad and always wonder when something is going to take a crap on you .That's no way to have a good reliable motorcycle that you can just throw a leg over and go at will. You know , we're talking about a machine that is almost 40 years old.

There's some indication from previous posts on this forum that the FPRs can weaken over time and result in a slightly lower fuel pressure in the rail. No suggestion that this can bring the bike to a halt, and the problem, if it is such, is only apparent when owners (including me) go noseying around with pressure gauges.
So no, I don't think that fatal FPR failure is a thing. 
As for the vacuum hose, just make sure that whatever you replace it with isn't thin wall stuff that could collapse. I use 4mm bore silicone hose which has quite a thick wall and that seems to be okay.

Yamaguzzi makes the point that, part of the pleasure in owning a K is in keeping well on top of things - they are well-engineered and can be stripped and rebuilt as many times as you like, most spares are available and this forum is a very sociable, informative and rewarding way of knowing that you're not alone in your spannering.

(Just mind the chronically off-topic halfwits, ahem  )

If two halfwits contribute does that then add up to one whole opinion?

If the diaphragm develops a leak fuel can be drawn into the rear most cylinder. In the worst case scenario this can cause a hydraulic lock resulting in a bent conrod. Before that happens you should get a warning of the engine dropping one cylinder and running rough. Checking the vacuum line where it goes onto the throttle body for fuel will indicate that the diaphragm is failing. The pressure regulators don't seem to fail that often in that manner.
Regards Martin.

They have to round up to call me a half wit...

I sort of half understand what you're saying...

Oh damn!

Point-Seven-five wrote:They have to round up to call me a half wit...

So are you saying that you're Point-Seven-five of a half wit?

Well I'm here to defend you, and I want to state that, no, you're much more than that.

daveyson wrote:

Point-Seven-five wrote:They have to round up to call me a half wit...

So are you saying that you're Point-Seven-five of a half wit?

Exactly.

The vacuum line at the bottom of the fuel pressure regulator is routed around to no 4 throttle body. You will see each of the throttle bodies has a little black rubber cap, no 4 instead of the cap has the vacuum line you describe. You should be able to get anything similar in an auto shop.

These are cheap, easily replaced [all push fit] and should be regarded as consumables because due to heat and location they dont last. Wise move is replace them on any K bike you buy.

I dont leave them on more than 2 or 3 years as they perish/crack and the bike runs like crap. The pipe will lead to bike running too rich and crappy.

While you are there you will see a Z shaped vent pipe that goes into the airbox, replace it to, it cracks too but you wont see the cracks and it also buggers the running.

PS We dont really have half wits around here but a few are cylindrically challenged for a missing cylinder. Then the full wits have to be nice to them because they really are the full shilling and have these nicer higher compression pistons that fit straight into a K100.....for more oomph. See, the K75 is actually a smoother ride....I dont own one but I did try to buy one and it was indeed a very smooth and impressive bike.