The faults of the Tešlov 344E and 344EE 4-6-4s, minor as they were, did still detract from the engines' usefulness. The rear bogies were prone to derail on tight yard curves and restricted the design of the ashpan, which tended to clog and impede the flow of air to the grate, affecting the steaming. In the early post-war years, the long wheelbase of the tender also caused problems on the indifferent track of the period. The non-standard 2000-mm wheels raised maintenance costs and crews complained that the big smoke deflectors obstructed the view forward. As soon as he returned to Ruhnia after the Second World War, Tešlov set about putting things right. Beyond draughting and pattern-making, not much got done in the event before Tešlov retired and the work was completed by his able successor, to whom the design is also attributed. Rodnivacek augmented his chief's concept, which included Houlet superheaters, bar frames and roller bearings, with standard-sized 1900-mm boxpok driving wheels, high running plates and improved suspension. In the final analysis, not much was left of the original engines. A total of 18 locomotives were rebuilt, some of the work being contracted out to Henschel, and formed a useful contribution to the RSR's express motive power stud.  Some locos ran for a number of years with Vanderbilt tenders, as shown below.

In the austere years following the Second World War, the RSR was looking for ways to improve the power and efficiency of its engines at minimum cost. The poppet valves used on a small number of LMS "Black Fives" seemed promising and were applied to six 344Fs in 1951. Comparative trials showed much improved performance and reduced coal and water consumption but also higher maintenance costs. Rodnivacek was convinced that through good detail design the poppet valves could be made sufficiently robust and trouble-free for general application. And so it was. The remainder of the class received poppet valves, conversion being completed by the end of 1953. Vanderbilt tenders were also applied to the entire class 344F, as shown below.