On a 1989 420SEL, how does one test the hydraulic lash compensators (HLC)?

I read the service manual and it said to press down on them and if one drops faster than the rest, then the fast dropping one should be replaced. But when I pressed down on the HLCs, they did not move a millimeter. Could they be filled with sludge or frozen?

I removed 4 HLCs, soaked them in gasoline, and they were still rock hard. Then I used a C-clamp to squeeze them down, several times. Now 3 of the 4 can be pressed down a few millimeters with force exerted by one's thumb. The fourth can only be compressed using the c-clamp.

Comments?

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1989 420 SEL