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Won't Hold Vacuum

rickas on Tue September 16, 2008 8:36 PM User is offlineView users profile

Year: 1972
Make: Chevrolet
Model: El Camino
Engine Size: 350
Refrigerant Type: 134
Country of Origin: United States

I had the stock A/C system completely disassembled and removed to clean. I flushed all lines, Evaporator & condenser. Installed new Expansion valve, drier & A6 compressor (rebuilt). I was very careful to lubricate new o-rings & use AMA Nyloc on threads. The system will not hold vacuum. I can achieve about 28in Hg but it will drop to 20in within 5 or 6 minutes. And is at 0 within half hour. I rechecked all connections and they are tight.

What can I do to locate the leak? Is it practical to pressurize the system? What would i need?

I suspect the problem to be in one of these areas: - Bad compressor (what a surprise that would be). I damaged evaporator or condenser while I had them removed ( the system functioned when I took it apart 4 years ago).

I'd rather not add 134 & dye until I fix it. Any thoughts would be appreciated. Thank you.


Rick Schaefer

73Wagoneer on Tue September 16, 2008 8:47 PM User is offline

I just installed a new evap and expansion valve in my Honda, and when I put it together I had a similiar problem, would vac down, but would not hold, I went ahead and charged it to see if I could find the leak, I have dye in the system, I ran the A/C about 10 minutes, I started looking for a leak (dye) but did not find one, I was forced to take everything apart again, still nothing, so I figured I would reinstall it again, after replacing the o rings the first time, I said why not do it again, I had plenty, while getting the new o rings, my old ones that I had took off where sitting there and I noticed that I had used o rings that were a tiny bit thinner, so after finding the right ones, I put it back together with no leaks, bottom line, even know I had dye in the system and a pretty good leak there was no evidence of the leak via the dye. I would recommend that you take it apart a recheck it, not sure if your system uses o rings, but make sure everything is checked twice

rickas on Wed September 17, 2008 1:35 AM User is offlineView users profile

Good point. If I don't get any other suggestions here, I was planning to dis-assemble and check it all. Which brings up another question.

I am using a Pre-8? o-ring kit from AMA. Its been apart so long I had no old o-rings to compare. I used the rings that needed to be stretched (slightly) to get them over the tubing. Should I have picked the rings that were loose on the tubing and had the same diameter as the flange on the tube? Thanx again.

Rick Schaefer

mk378 on Wed September 17, 2008 9:17 AM User is offline

Rings that are loose on the tubing are going to be a problem. The ID of the ring should touch the tube but not require extensive stretching. More important is the thickness of the ring.

HECAT on Wed September 17, 2008 11:51 AM User is offline

Stretching an o-ring will reduce the diameter and change the sealing ability.


HECAT: You support the Forum when you consider for your a/c parts.


iceman2555 on Wed September 17, 2008 2:19 PM User is offlineView users profile

When servicing a vehicle that looses vac this rapidly.....first thing to check would be the connections on the service equipment.
Also, why not simply inject sufficient refrigerant to test the integrity of the system...guessing what and where is not a accepted procedure.
This is a straight forward problem...and simply requires a direct approach to a solution.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

rickas on Wed September 17, 2008 3:01 PM User is offlineView users profile

I did leak check the gauge set, closed the check valve near the service connection & it held vacuum overnight. This AM I disassembled all connections & replaced orings with the best fit I could find, Still leaking. I'm gonna leave it be for right now. I need to reassemble the front end & get it inspected before the end of the month. When I get back to it I will add dye & a small amount of 134.

Rick Schaefer

mk378 on Wed September 17, 2008 3:28 PM User is offline

You need to have a system fully charged and running for dye to work. Kind of a waste when you know it leaks anyway. By using a small amount of R-134a you just have enough to build positive pressure in all the lines but don't run the compressor. This only takes a few ounces. Then ideally look for it with an electronic leak detector, or soapy water can be used on the connections to see if it bubbles.

Edited: Wed September 17, 2008 at 3:29 PM by mk378

73Wagoneer on Wed September 17, 2008 8:59 PM User is offline

If its an o ring leak more than likely you will need a sniffer, and to make the point again, the thickness of the o rings matters, sorry you don't have the old ones to compare them to.

cam76034 on Thu September 18, 2008 12:48 PM User is offline

I'm having the same problem. After replacing the expansion valve on a R12 equipped 91 Mazda Miata, it won't hold a vacuum, although before I replaced the txv it would hold. I've gone through and replaced the o-rings I had to replace twice now. Since I've got a limited supply of R12, how do I go about finding the leak without wasting all my R12?

mk378 on Thu September 18, 2008 1:12 PM User is offline

Use a gas other than R-12 for static leak testing under pressure. For example, air (must be dry), nitrogen, or R-134a. Always pressure test a R-12 system before the final evacuate and charge with R-12.

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