How do I service my Shindawa SP4115 sprayer which leaks?

This back-pack agricultural sprayer for professional use with Roundup has started to leak pressure. I estimate the chemical has rotted the valves +/or seals, which may need replacing. Since this probably is a comoon problem, is there a repair kit for this model?
Also, I have the same problem for an older sprayer the RL ProFlo XL Model 1997H, 3 Gal. I believe it is by Root-Lowell Mfg Co of Lowell MA 49331