Set To Recharge AC, Help With Facts

[razorboy]

Hey guys, gals,

I am getting set to replace the Schrader valves on my X5 AC system and just want to offer a little obtained knowledge and do a little fact checking before I do the work.

The AC guy I have been working with claims that the majority of problems on AC systems are leaking Schrader valves. He did my little Fiero project car because of this and now the AC runs like a champ. My X5 also has a non-op AC system and when I pulled the cap off the high pressure side valve, it was hissing. Now there is an o-ring in the cap itself but this will leak in very small amounts until the freon level drops below the low pressure safety switch threshold and that stops the system from working. There can still be pressure in the system, just not enough to operate above the switch.

For those of you who have cold air on drivers side and war on the passenger side, this I have learned is the way BMW designed the system to work. When the pressure starts to drop due to a leak, the passenger side will stop blowing cold first.

I also discovered that my particular MY, X5 uses a different type of Schrader valve and found the normal ones for it on eBay. Parts should be here today. What I started with was unscrewing the cap on the high side a little until I heard hissing. I did this to allow the 134 to vent out since the next step is to remove the current Schrader valves and replace them with the new ones on order. I also wanted to drain it slowly as to not lose any compressor oil. Hopefully that works out.

Here are my questions:
The lowside Schrader valve is the one closest to the firewall correct? This is where I will recharge from. My new schraders have colored caps so I assume the red cap is for high side and the blue cap is for lowside?

I have read the system holds 1lb of refrigerant. How is this determined on the cans of 134 you buy from the shop? I have only seen pressure readings on these cans with no way to determine for sure how much by volume has been added. Are the supplied gauges on the cans trustworthy? I have seen that 45 psi is supposed to be the range you want to be in. I won't have gauges to check both pressures high and low so I just want to know how much faith I can have in these cans for an accurate refill.

Cheers
Bernie