Ive owned dozens of HW35s in both .177 and .22, various barrel lengths and ages (1968 - mid 80s). I presently own 17 of the things. Perhaps Ive just been lucky but Ive never had one which has suffered from 'breech leakage' which is often mentioned on here. Worn barrel catch (and cylinder face), breech seal (early ones were leather and deteriorated) - yes, but leakage, no.
I find it difficult to understand how a small cavity in the cylinder isn't filled with gunge, dried grease and leather bits (off a leather main seal) over the years of usage.

ATB
Ian