i have BRF28.09 Model concrete pump i tried to adjust it's stroke number but i can not (according to data sheet it must be 28 but it is now 21),although i adjust the following according to it's data sheet :-R.P.M,new acumulator pump,acumulator precharge pressure and cut off pressure also i checked the connections according the hydraulic circuit of the pump what else i have to check

Are you in the correct PTO gear? (usually second from high gear) Is the pump getting the proper pressure signal sent to it to give it's maximum stroke?

Do you have the Ser # I can check into the spec sheet.

Hope the hopper is wet or it'll wreck the kidney seal.

 I have to ask but are your filters brand new? And what is the oil temp when you are performing the test? I ask because I once had a mechanic or supposed one readjust my system with dirty filters and cool oil. And as Teleabelt said you better at least have water in the hopper, not only does it save the kidney seals but, it also lubricates they and, the mudcups. Stroking it dry sets up alot of resistance. 

If you are in the correct gear, the tack and the speedo will look to be in the same direction