I still have R12 in my system but with a gauge set both high and low are reading 30 psi with the compressor running. I only have a short supply of r12 left and not ready to convert to r134. Is this and indication of a bad compressor or should i risk putting in more r12 and see if the high side reaches 200 and the low 30? Any other way to check the compressor without opening the system? Thanks.

Doesn't really sound like "low on refrigerant" symptom.

Sounds like either you're just not getting a good high pressure reading for some reason or the compressor is not pumping.

That's what i thought. It looks to be the original compressor. Time to change it out. Thanks Dean.

If you aren't going for originality, you might try the replacement 10 cylinder bolt on compressor, Vintage Air or something like that sells them, good luck

If you're going to change out the compressor it would be a good idea to flush the system replace all O-rings and install a new expansion valve and also the dryer bottle.
Also while doing this it would be a good idea to remove the inner half of your AC suitcase remove the evaporator and clean out any debris that is in the suitcase that has collected over the years so you can get a good airflow through the evaporator.

I just finished installing my stock system on my 68. While charging I was reading 50/70 up to the third can. I changed everything except for the compressor during my restore. I used R12a refrigerant to make sure all was well. It wasn't until the 4th can that my pressure changed to a more normal 38/160 psi. I wasn't sure if something was stuck since I changed the POAST, expansion valve, condenser, dryer, evaporator and all lines and clamps. As stated i did not change the compressor but I did fill the reservoir and manually spun it to see if it pumped air and was quiet which it was. You are going to have to collect your R12 and not allow it to just be released to the atmosphere so adding a little more will not cost you if you follow the law which will save you $ anyway. One question, are you sure the clutch is engaging and not slipping? Ok two questions, is it loud making noises like something is broken?

Not really necessary to replace the expansion valve, just check and clean the inlet screen.

{EDIT} that's if the type of refrigerant is not being changed.

it spins freely, clutch engages well. No noises at all, sounds fine but no high pressure. Should i just add r12 and give it a try? I have about 13lbs left in my 30lbs can. Dont want to waste it! If i remove the hose connection on the back of the compressor and spin it should i feel suction If the compressor is working? What other test can i perform? Thanks.

You still have pressure in the system so it has not been contaminated by the atmosphere. Check your system for oily/dusty spots to make sure you have no leaks. Check the front of the compressor and clutch for signs of oily /dusty accumulation which might indicate a bad seal on the compressor input shaft.

Charge it. At 30 psi you won't see high pressure build because Freon is being released at that pressure through your condenser.

IF the service gauge port valve core is being opened by attachment of the hose and the gauge manifold valves are closed, and you a truly getting high pressure reading to the gauge, the high pressure should always be higher than the low side pressure IF the compressor is pumping.

I've seen and replaced many compressors with bad valves that have that same symptom.
However I must say that none of them were that exact same type of compressor and I'm not really familiar with how that particular acts with bad valves so...

Yes, he's in a spot where it's a crap shoot. The only way to find out if it will work is charge the system. Once he gets up in pressure, he may find leaks or the compressor is shot and won't produce sufficient pressures. Without a charge in the system diagnosis is difficult at best.

The only sure bet involves replacing most old parts with new, flush, vacuum, leak test and then charge.

Ok, changed out the compressor and that fixed the pressure issue of being equal. I have about 32 on the low side and around 200 on the high side BUT the high side is jumping around between 200 and 250 very quickly. The low side is steady. The inside temp is around 52 degrees. What is causing the high side meter to jump so rapidly? Thanks all for your help! I think im close.

Sometimes the quick fluctuation of the high side gauge needle does that if there is not an orifice in the gauge or somewhere in the flow path.
It can be stopped by throttling down the flow by almost closing the service port but of course there is no way to do that on the car.

I would place a fan in front of the radiator or spray a little bit of water on the condenser with a garden hose nozzle every now and then to lower the head pressure while charging the system and try to get the low side pressure up to around 36 - 37 psi with the A/C on Max high and the car doors open.

Did you clean the expansion valve inlet screen?

i did clean the valve. I'll give that a try with spraying the condenser wtih water and getting the low setting to 36. Is there a danger zone for the high?? Over 300?

There should be a high pressure pop off but if you sprinkle a little water on the condenser the head pressure will drop drastically.