What up transmission won't go in reverse when first cranked up until after 5 min

1. Slow, slipping or no reverse: “lo-reverse” clutches are worn out, fluid leak in the reverse apply circuit, or broken sunshell. It is possible to remedy a fluid problem by removing the checkball from its cage in the case in the rear of the transmission may help (must remove valve body), or adding a high-viscosity additive or other seal restorer product. May also have worn boost valve (can replace in the pan).