>> Sounds like it needs a proper service as you say but it's nice to know a little about what's involved beforehand to avoid being fobbed off!
and other post:
>> I also agree with you about the cans of gas available at Halfords etc. I always advise people against using these for the reasons that you have stated.
I think the thing that would clinch it for me is that the halfords topup will be 30 quid (and every likelihood is that just one 300g tin isnt enough - my recharge weight on my Omega was 950g or so off the top of my head). Compare this to a vac down, recharge with the correct amount plus maybe dye costing 50quid or thereabouts - its worth the extra.
>> How long should a system have a vacuum on it prior to a refill with gas and lube? I was always under the impression that the longer the better but have since been advised that this is not the case. Which is correct?
Hmm, the answer to that all depends on 1) if moisture is present as this takes time to come out of the system and 2) if you have a decent enough gauge to know when you have reached the end point.
1) A good level of vacuum takes time to reach, even with a decent vac pump with new pump oil.
2) Boiling point of water reduces as pressure reduces (table in link below)
3) Water in the system then starts to boil off at the low pressure
4) But this takes time to boil off and be pulled out of the system.
(Long pipes and slow moving molecules!)
5) Water presence & boiling off indicated by a long time to reach a good vac level
(A leak is indicated by an inability to hold that vac level with the pump isolated)
If you DONT have a micron level gauge then I'd say that in general longer is better... Most people will accept something between 20-40 mins as being about right, but this figure depends on how long it takes to boil off and pull out any water. Shouldnt be much water there unless the rec/dryer is saturated.
Better is to use a Micron level gauge so you know when to stop! (Which may be why some people say that longer isnt always better - and with the right kit this is true.)
From Robinair (Auto vac pump site)
http://www.robinair.com/acsolutions/acvacuum/acvacuum.php"best way to know for sure that the system has been thoroughly dehydrated is by using a thermistor vacuum gauge. This prevents wasting time by pulling down after evacuation is complete or risking inadequate dehydration"
On my car with no commercial pressures of time? I would wait until I reached max reachable vacuum on the micron gauge (indicating all water boiled off) and if that value was acceptable - I'd then hold that for about 10 minutes to be sure. This would take say 30-45 minutes on the pump in all if the system was generally OK.
Hope this helps,
Jon