Ktm 525

Guys I’ve got a bit of a curly one with my 07 525. I last rode the bike about four months ago and went to start it after a service I did two weeks ago and it would not start. After a lot of kicking it eventually started and ran fine but as soon as I put the crankcase breather hose back on the carby it cuts out and it is really hard if not impossible to start.I did a cylinder leak down test and it has 85-90 percent leakage past piston and 65ft pound comp…yes I know what your thinking rings piston etc and that probably is right but how does a bike with 1300 ks and is over serviced and babied get so low comp….very disappointing. Anyone shed some light on problem.