We have the twoo tractors and both loose their fuel pump primes if they sit for more than a day or two and the fact I can't track down where  the air leaks are is driving me nuts. 

With both 3 - 4 pumps on the hand primers and they start right up, followed by a few seconds of sputters and surging while they push the remaining air out, but if I don't they are air locked hard and will not come out of it on their own. 

SO what am I missing? 

is there a common seal or something that ages and starts working like a check valve keeping system pressure in when I try and test it but lets the fuel drain back out after sitting for a  day or two or what?   

I've power washed them clean in order to look for drips or seeps and I got nothing even after hours of running yet after a day or two of sitting their pumps are air locked again. 

I was starting suspect it may be a fuel line issue but after the washdown and wait the fact that I never got so much as a noticeable drip let alone damp spot had me ruling that out and that there had to be some seal or O-ring I wasn't finding that was giving me the problems.    

I didn't think about there being a dip tube on the return line but that is worth looking at as well. 

Thanks!    

(Now if only one of you guys only knew the working pressure range for my MF 2745 multi power hydraulic circuit so I could replace that bad Hobbs switch I'd be almost be set on the bigger tractor repair work for a day or two and could get into the smaller tractors next. )