Found under the following brands:
Master Tool Repair
This kit contains the following items
B5900/B5000 Valve Plate Assembly. NOTE: This assembly is for 2 PIECE VALVE PLATE STYLE ONLY). Item comes in 2 separate pkgs. B5000 and B5900 pump now use the same valve plate. Poor performance or air leaks out of the intake filter while the unit is running? This is probably the problem.