Help needed please I have a hayter ride on mower with a 13 briggs stratton engine .it will run OK without the air filter and breather pipe connected but as soon as I refit the air filter ,plug rubber breather pipe in place and then it won't start .if I put my thumb in the breather it won't run .I have checked the breather disc and it looks OK but mabey I'm not sure what to look for .any help

The breather just lets internal crankcase pressure out.. but let me get this right... when you put your finger over it with the air filter off, it won't run?

Yep if I put my finger over the rubber tube and try to start the mower it won't start

You can try replacing the breather along with the tube... and see then what it does... while your there, make sure the valve springs are not offset, and no holes are plugged.