I have a nice little Falcon ladies bike, it sits outside and resists rust amazingly. So one day as I was out and about it got a flat so I wheeled it home and sent for a new tube, it was a 26" and the outer stated 700 x 40 [plus some other numbers in parenthesis], anyway I kinda got it on and blew it up, next day it was flat. So I sent for a 700c x 40, but it came as a 700 x 35 ...[ 40]? but on the tube it also said 28" These are good tubes: [Swalbe] It seemed to fit the outer tyre.
With some cursing and difficulty, I managed to get the outer off and replaced the inner, but again it was flat in the morning.
The valve passes the "spit" test, and the previous new inner[the 26"] has no puncture . There is no thorn or cut in the tyre. The valve cap is fitted.
I use a Zefal hand pump [very hard work] , the bike had a short test drive last night, it held air during a 200 metre test ride.
I feel a bit of an idiot, having fixed numerous punctures, but these bigger tyres are hard work compared to racing tyres.
https://www.ebay.co.uk/itm/SCHWALBE-700-x-35c-Inner-Tube-40mm-Schrader-Car-Auto-Valve-AV17/253290718619?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649