These are links which show how pressure test is done. As long as there is no bubbles, the head is good to go. It may not be necessary to test with hot water. Please understand hairline cracks do not mean cracked head. One word of caution for any one who tries this. Wear safety goggles. If the glued coin popped under pressure, it can fly anywhere.

https://youtu.be/A26KfDj6wAM
https://youtu.be/VJL-hcC_CBA
https://youtu.be/x_OHmyTYDBQ

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.