Thanks Sam, I’ve found a hack that makes the Reba RL works (ish): No impact on the ‘open’ compression damping, but at least the lock-up works as intended (almost fully locks-up after 3/4mm of travel).
I replaced the spring at the bottom of the compression damper assembly with a much stiffer spring (not quite the right dimensions, but it works). It’s the spring that maintain the blue-anodised valve closed during compression and open during rebound.
Compression damper assembly
Valve in open position (during rebound)
After a ride, the modification seems to works consistently. I’ll see over time if I try with a higher viscosity oil, but I’m concerned it would slow down too much the rebound damper.
For now, this will do the trick. I wish someone at Rockshox will read this post and apply the modification to the gazillion different versions of the Reba RLs, or at least offer a replacement spring for their exiting customers.