potential leak prior to evacuation/recharge - how to find

[aribert]

61 Ford Falcon
64/64 Mustang underdash unit
11x20 parrallel flow cond
Sanden 508 comp
Parker filter w/ U.A.C. GY209 press switch (see images)
R12

I replaced the R/D and binary switch, low press hose asm from comp to firewall and the expansion valve. The underdash unit (and expansion valve) has flare fittings, the balance of the system is o-ring. I added 1.3 oz mineral oil to the R/D inlet port prior to reassembly last Friday. Yesterday I pressurized the gages for an hour to 150 psi with nitrogen and the needles were steady. I pressurized the system to 150 and saw a drop of 10 psi in 2 hours. I checked every joint that I touched with a leak detection fluid (intended for plumbing; nat gas, propane, etc) twice. No visible leaks. Left for the day somewhat dejected. Last night I got to wondering, is it possible that the oil added 3 days prior is still flowing under pressure into the desiccant in the R/D? I'll see what the remaining pressure is when I get back there this afternoon. Since I do not observe a noticeable leak, should I just start to evacuate and see if the system holds vacuum?

[Cusser]

I'm NOT a pro, so keep that in mind here. I still use R-12 in one of my trucks.

Since you are using R-12 (expensive, though plentiful and available), I wouldn't add any R-12 until a nitrogen leak test AND vacuum-holding test show no leakage.

I've heard of folks adding a can of R134a after evacuation (WITHOUT actuating the compressor) so electronic sniffer could check for leaks at the evaporator (vents) and compressor seal. This can be done without any special R134a equipment or fitting adapters for your vehicle. Then the R134a gets recovered and the leak fixed before adding R-12.

Wait to see what others say.

[bohica2xo]

Leaks at service valve connections, service hoses and the test manifold are not uncommon.

I usually screw the high and low side hoses to a tube union and pull a vacuum on a gauge set as a test.

[JohnHere]

A system that holds vacuum is no guarantee that it will also hold pressure. Did you try leak-testing the system using an electronic detector ("sniffer"), generally more sensitive than other methods?

You pressurized the system to 150 PSI using nitrogen the first time (pressure held). Then you pressurized it to 150 PSI using R-12 (??) and the pressure dropped 10 PSI in two hours. Am I understanding that correctly?

[aribert]

A system that holds vacuum is no guarantee that it will also hold pressure. Did you try leak-testing the system using an electronic detector ("sniffer"), generally more sensitive than other methods?

You pressurized the system to 150 PSI using nitrogen the first time (pressure held). Then you pressurized it to 150 PSI using R-12 (??) and the pressure dropped 10 PSI in two hours. Am I understanding that correctly?

NO. I have only pressurized the system with Nitrogen. Pressure dropped 10 psi in two hours and continued to drop to 75 psi over 22 hrs. I have not pulled a vacuum and I have not attempted to charge the system. I have a TIF 5550 detector - I have never completely trusted it - it seems hypersensitive. I just did a quick search and there appear to be replacement tips available or a calibration service for the 5550.

Twice I leaked checked with fluid on the outside of all connections that I disturbed along with hose connections and even the valve stems on the service manifold. I pressure checked the manifold/hoses first (before pressurizing the A/C system)and they held 150 psi for 2 hrs.

[aribert]

Leaks at service valve connections, service hoses and the test manifold are not uncommon.

I usually screw the high and low side hoses to a tube union and pull a vacuum on a gauge set as a test.

I pressure tested my gauge set at 150 psi and the pressure held steady for 2 hrs and I deemed that to be OK.

[bohica2xo]

That takes care of the low hanging fruit. Do you have or can you borrow a sniffer?

You can as has been suggested add a couple of ounces of R134 and pressurize the system with nitrogen to 150psi for the sniffer.

[tbirdtbird]

With extreme respect to Brad, FWIW I was always taught not to pressurize the evap to more than 100 PSI.
IMO the electronic sniffer is the most accurate. Is there a sensitivity setting on the TIF? I have a TIFF XP1A and love it.
When you use a sniffer, you have to be sure you are getting reproducible readings at any given location. Using a can of computer air, gently blow off the sensing element (from a distance, you don't want to get the element cold). It may be contaminated. Shop air has oil in it, that is why you use computer air.
Be sure to sniff the evap drain tube. Put a shower cap or equiv around the comp and let sit overnight and stick the nose of the sniffer inside. This assumes you installed a few oz of a refrigerant into the system and then used N2 to pressurize. I usually puff in enough refrigerant to get my low side up to 20 psi, and I don't even bother with more pressurization. But since you have the setup, you may as well

[JohnHere]

Did you use Nylog Blue on every connection point that you disturbed? If not, it's well worth it—at least in my mind—to re-seal each of those connections using new gaskets and o-rings, if appropriate, and the aforementioned Nylog Blue, which will help seal the tiniest of leaks. A little goes a long way, and it's compatible with all refrigerants. Then, repeat your leak-testing procedure.

[bohica2xo]

T Bird:

Check your P/T chart for 134a

At 120f the vapor pressure is 171 psi. Evaporator sees every bit of that in a car parked in the sun in places like Phoenix.

So 150 is about as high as I like to go on an MVAC system. R410 in residential will make you flinch a little more.

100 psi is fine. OP was already going to 150 and seeing a leak rate that a sniffer should find.

You mentioned R152 - keyboard duster. How well does your Tiff pick that up?