To be clear, the W210 was sold in the US in the 96-02 (‘03 wagon) model years. The W211 was sold in the 03-09 model years.

In the 210, the check valve an integral part of the pump. You toss a good pump to replace the check valve. A bad check valve only affects starting. The engine runs fine once system pressure is up. Starting is difficult because the sequence runs the pump for a couple of seconds then waits for an rpm threshold to run the pump again. In a 210 you can turn the key from off to run without hitting the starter a few times to build pressure. I don’t think there’s an equivalent with a keyless system.

Sixto
98 E320s sedan and wagon