Hi Richard, I have used that product and really hoped it would work as described.
It sorta did.I offered it to my customers on a test basis. as the maker warranted that if it did not seal , they would refund the cost of the kit.So I tried it maybe 10 times.
On 2 124 cars we put a new evap in after the sealer failed to stop the leak.Similiar results on 85 123 cars. (it seems that 85 123's used a junk core too. they all leak) On a BMW it worked permanently. On other 124's I have had problems with compressors leaking after wards, drier switches failing repeatedly, exp valves leaking.I no longer off to use the SSR cryo chem product.
The stuff is very hard on the equipment used to install it ,too. If it gets into the shops recovery machine they will hate you!
On 124's the evaporator is a common failure, and very pricey to replace.The dash comes out, HVAC box is totally disassembled, etc.
I also replace all the vacuum elements while the unit is apart.
If you intend to keep the car ,I would bite the bullet and replace the core. I did on my wifes car, and nobody hates to spend money on a car more than me. But, when you gotta do it....

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Michael Haven CMAT,CMM,L1
MPH Automotive Services
16510 Park Row
Houston, Texas, 77084
281-579-8885