Need help tracking down AC leak

[Terrorizer]

The heater on my 1990 SHO isn't working because of a leaky heater core that was bypassed by the previous owner. Recently my AC stoped working as well, based on the symptoms, and the fact that the previous owner had the entire AC system replaced a year ago, I am pretty sure the problem is an AC leak. From what I understand it would be best for me to identify the AC leak before pulling off the dash to replace the heater core, that way I would know if I need to replace any possible leaking AC parts back there. I took my car into a local shop to get the AC checked for leaks, and they said the AC was leaking where the manifold meets the compressor, and he suggested I start with replacing the o-rings. I have a few questions:

I am familiar with the compressor, but what/where is the manifold?

Also, I asked him if he checked the interior of the car to see if anything behind the dash was leaking. He said there was nothing back there that could leak so it didn't need to be checked, is this true?

The shop gave me a weird vibe, it kinda seemed like they just wanted to get me in and out of there, but they didn't end up charging me to check my AC so at least I know where one leak is. I would appreciate any info.

 

[projectSHO89]

The "manifold" is the block that mates the tubes to the compressor.

The evaporator is onside the dash and is a potential (but very rare) source of an AC refrigerant leak.

Since the evaporator has to be disconnected (system evacuated first) in order to replace the heater core, you might as well replace all the o-rings in the AC system while you're at it.

 

[sperold]

As Above
A shop that doesn't charge you for a leak inspection and wants you in and out fairly quickly should not be feared.
A lot of people call any of the rigid piping that bolts onto the compressor the "manifold".
These parts are the hard to find parts as they are no longer manufactured, the flex lines are still being made.
There are usually signs of oil where the leak occurs.
Yours was a Freon 12 system (if not updated). Look into replacing it with freon 12 but it may be difficult to do. The replacement freon is easy to locate and claims to be compatible with the R-12 seals, but look into it. The expansion valve may have to be changed when you go from one to the other (if it is not already done).

 

[Terrorizer]

The previous owner converted it from the old type to the new type, but it sounds to me like I should still have the inside part checked just to be safe. I would hate to pull the dash out, replace the heater core, replace the o rings and have the system charged, just to find out its leaking behind the dash and have to go through the whole ordeal all over again.

 

[projectSHO89]

The evaporator core can be tested in situ with a vacuum pump/gauge and some ingenuity.