So on my 1995 Cherokee the a/c cuts out and blows out defroster when I'm drivin in high rpm's. Yes it's a vacuum leave and I found that the grey line that runs through the fire wall to the heater control valve and he black lime that runs through firewall to the "T fitting" needs to be replaced. How do I go about replacing those 2 hoses since they go through the firewall and I need to replace the whole hose?

 

Here:

 

The grey and black lines go into the firewall, how do I replace the whole line

 

Sure you need to? The issue is usually in the engine bay.

 

Possible bad check valve ....

 

Yeah, mine does the same thing. Usually after speeds of 40mph or uphill. Sucks too cause I work over 1K elivation higher than where I live in Vegas. In 2015 when I moved out here I had to drive my Jeep to work for a month straight and in May and June so it was 90's and 100's with no AC. I eventually found this little nifty trick to get the AC working.I start by setting the cruise control to the speed I want to go, then speed about 5-10mph over that speed, shut the AC off and select 1 for the fan, turn the AC back on as the Jeep is decelerating, then bingo, ice cold air comes out. Then I can turn it back to whatever fan setting I want. If you start accelerating again or go uphill enough to get the RPMS past 2 it will cut out though and you have to do it all again. I just started searching this issue a few days ago after having it happen to me for a few years.

Here are some links that people talk about some issues with the vacuum pump. Hopefully some of it can help you find a fix. This is on my next to do list. The thing looks like one of those ball things in the toilet tank that floats to cut off the water pump when it fills after you flush. It is on the passenger side under the battery. Cruser54 posted a solid picture that lines up exactly since I just followed my liner earlier today. I wish I would have found his post before I looked lol. You can get to it by removing your battery and battery platform like some people say. I got to mine by removing the plastic fastener things down under the bumper (you would be looking up towards the wheel well for the round black plastic button thing). Just jam a flat head under it to pop it out and you can pull back the plastic area that covers the vacuum. I looked over everything today and nothing seemed bad. I followed the line and everything appears plugged in. I will try again one of these days and maybe try testing the vacuum pressure like some people said to do. Also, someone said it that vacuum is cracked it could be the cause?
https://www.cherokeeforum.com/f2/hel...eration-50480/
http://www.jeepforum.com/forum/f11/c...lerate-158984/

Here is a picture of what the little bugger looks like:
https://www.google.com/search?q=98+j...ZhzpsrSWftEFM:

Also, just found a good way to test for leaks in the line. This guy says "The source vacuum from the manifold only brings vacuum to the HVAC system, the purge system and the Cruise Control (if equipped). I doubt(my opinion) if a tiny leak in one of the small pass. side hoses would cause a running or driveability problem. But it will cause the blend doors to default to defrost mode. A cracked hose, a loose connection, a tiny wear or corroded spot, especially under the battery tray where the vac lines dive down to the canister are all likely suspects. It's fairly easy to find a vac leak on a 4.0. Blow a little smoke (and when I say smoke, I don't mean from a smoke machine, invest in a 1 cigar) into each hose...see where it comes out = leak. REPLACE any questionable lines. That'll fix your HVAC leak."
http://www.xjtalk.com/showthread.php?t=10555

Do please post back if you find your fix. I will post back if I eventually get mine fixed along with whatever I do to test/fix it.