I want to know if somebody use K carbonate to rise D-max for print. I ask because I read that for LPD developper, the blacks are not so good than for the Dektol.
The secret to getting good blacks starts with a properly exposed and developed negatives.
In addition some people pull a print before it has completely developed. This prevents blacks from developing completely. Follow the manufacturer's recommendations carefully with respect to time and temperature. You shouldn't have to add anything to the developer. You might also search APUG for discussions on developing prints to completion.