Fuel Selector Valve Rear Engine

[gcball]

Last year I had my rear engine rebuilt for my t337b, with new everything. During the rebuild the mechanic notice that aft fuel selector valve had corrison and recommended that since it was original to send it out for rebuild, I reluctantly agreed (it worked fine). After about twenty hours on the new engine a month ago I noticed a fuel flow problem on the rear engine when switching from to the aux's (cruise flight at 7500). A couple of minutes in to the aux's tank (rear only) and the fuel flow started going erratic from 13 to 5-6 gph. Boost pumps would not always fix the fuel flow problem but, switching back to mains would. This is only happening on the rear engine. After flight, mechanic/shop troubleshot and found that the new fuel pump for the rear was not up to par and replaced (in warranty), thought this would fix the problem. Two weeks later same scenario, flying at same altitude on mains and switches to aux's with the same rear fuel flow problem. Tried to troubleshoot pumps would'nt overcome problem, landed and had same shop troubleshoot again. This time they said it was the aft fuel selector not responding properly and sent it to B&S for eval/repair (by the way, B&S Repaired and Certified this same aft selector in May of last year-see above). I have since sent it back a third time and apparently B&S although they charged $90 for bench tests did'nt test it and said they would'nt charge our shop for the test. My question is, is this the fuel selector valve completely? I have seen the valve and understand that it's pretty simple but, tomorrow will be the third try to get the valve to work properly (last time we could'nt get it to shut off). Are these valves tricky, how about the linkage, any tricks to setting up. I'm spending (and hope to recover) a lot of labor hours taking this out and putting it back only to be in cruise flight with my family to find that when switching for the mains to aux's the rear engine wants to quit. Any thoughts?

Thanks in advance,

Gavin Ball
T337B-0637 a.k.a Tweetie Bird (Third generation in this plane)