I have a clunk in the car when the car downshifts. From researching this forum the conclusion seems to be a bad check valve (on vac line from pump to brake booster). Either this or I will have the mechanic look at the whole vac sysstem to find the leaks. How much does this service cost?Is it necessary to do this repair?I think I can live with the ,but if it ruins my tranny, this is a whole new thing all together. What's your twocent's??? Gracias. amigos!!!