Frustrating lack of 'Bulb' ability. Help!

I bought both a Miranda Sensorex II and Fujica ST 701 for only about $30 total. Both look great and have had no apparent abuse. But...the instantaneous status of the 'bulb' setting (agruably the most important shutter speed?) is very frustrating. (I would sacrifice X sync in its place!) The Miranda does sometimes work correctly at 'bulb' if, and only if, I press the shutter very, very quickly. It's like there is a two stage mechanism whereby something has to kick in before the second curtain gets a chance to close. I have flushed out the shutter gear mechanism with lighter fluid to remove potential dirt and still it is this way. I have a feeling that there must be a hidden spring that is not catching the second curtain fast enough but, but, but...what to do? I do minor repairs like changing curtain tensions and cleaning the prism area. On these SLRs does anyone know how to explain what 'wears out' causing 'bulb' to become instantaneous? Thanks. - David Lyga.