1982 310a bucket curl drift

i have owned a john deere 310a now for about 4 years,and for it,s age it has served me well for household duty,s.the biggest problem i have is most all of the cylinders drift fairly bad,especially the bucket curl. i have had both of the cylinders professionally rebuilt and have been told that they are in excellent condition,and have resealed the loader valkve section myself,but still have really bad drifting cylinders,either running or not.i was wondering if there might be a way to add an external one-way valve in one of the return lines to stop this from happening,without having to spend thousands on a new spool assembly,which i geuss must be worn,although it did not seem that way when i resealed it,any input or suggestions would be breatly appreciated,thanks for looking. s barrett