727 in reverse issues all of a sudden rpm w/lba

hello i recently ran into issues with transmission.. when put into reverse acts weird.sometimes it will back up normal other times ,you have to give it alot of throttle and when it does move, it like chatters the tires.like its fighting itself?then sometimes it will back up with barely any throttle.ive done the following dropped the valve body,Air checked front rear clutches and low/reverse servo and front servo everything is applying and releasing.adjusted low/reverse band. i can drive the truck up and down the road and all forward gears shift as it always has.any ideas? tranny is built with following cheetah rpmvb w/lba added later by john cope,red clutches, kolene steels,both bands were new and red lined material.4.2 lever.hd spring and extra thick rear servo cover,upgraded reaction support oil mods,i have everything written down with all specs from when i rebuilt it. thanks

Call him, CRT, and ask him for his help
Are you sure the shifter is adjusted to dead nuts on? If not look at that

Mine did something similar when the brake button was messed up .., took it apart and cleaned it and was back to normal

I would be interested in knowing how he did this. Anybody know? Does it affect the crisp shifts of a cheetah vb? I guess more importantly, could this mod be what is causing the o.p.'s problem?

I had John modify a Cheetah valve body to LBA for me. Its worked fine. Does 3rd gear act normal and acts up only in reverse? To me either the rear band is slipping (damage rear servo). Or something is applied that shouldn't be. Either by valve body issue or possibly a sticking rear clutch. Broken belleville spring?
Doug

this is not a fresh trans build,i built it roughly 4 yrs ago but only has maybe 350 to 400 miles.the trans has always shifted fine and reverse worked as normal.drove to work other day everything was fine.went to back the truck up in driveway other day and reverse acted funny.all forward gears shift as it always has.i looked at the rear servo with a flash light and didnt see any damage.i did not not remove the servo too look at it.applied air pressure to l/r servo and it applied and released.held the air pressure to it and did not hear any leakage.while i was there i re-adjusted the l/r band also.the valve body was removed while doing all this.

acts up only in reverse .yes it shifts great into 3rd just as it always has.all other forward gears shift nice and firm.just looking at ideas before i drop the pan again and drop the servo out to inspect it and the seal on it.i checked the shift cable linkage and shift lever position but will give that a db check also before dropping the pan to eliminate that ,it a b/m pro ratchet with super duty cable routed along ways from any exhaust.thanks

I would check the servo piston and pin for binding. Sonnax has a steel pin for this scenario. I personally haven’t experienced this, but it seems to fit. Since it worked before, I wouldn’t focus on the valve body, unless you had it apart, as it was functioning correctly. Good luck!

checked again and the shifter in reverse and the trans shift arm. in reverse adjustment is dead on.the pin drops right into the shift arm.i ran shifter forward and backward and it drops in everytime.so that is eliminated out.should i do a pressure test on the low /reverse servo port with it cold and hot and see if it drops in pressure ,before i drop the pan?