One idea is to try another servo as the clip on pump end may be worn ,the valve is usually approximately 8mm ,I measured one here and it was 7.5 mm.
You can just grab a rag and put it around the valve before putting the pump end on the valve, if it's still too loose fold the rag over twice then put the rag over the valve ,it just depends how thick the rag you are using is .
Probably not a bad idea is to buy a bike pump with a screw on end instead of the clip on type , they can be bought at Amazon or Kogan.
