For the guys saying they hate real estate agents, maybe rethink it to "I hate the way some real estate property managers are towards tennants". Property managers are there to take money, protect the property of their client and to make arrangements for repairs and maintenance. No more, no less. No need to hate the person, just their actions. They are, after all trying to look after their clients $300-400,000 property which could well be their only fall back in retirement.
No sense in getting wound up, bitter and twisted just yet.
Take a breath.
That said, it sounds like a case of they have no idea. A Property Manager can't stop you from having a small scale brewery in a garage if it's not doing damage or at risk of doing damage to the property. So....good ventilation (Steam damage) , no dodgy plumbing add-ons, no stains anywhere, no holes in the walls, no risk of a fire, no wiring changes etc should not present any issues for either the PM or the owner if they understand the situation and understand that home breweries are perfectly legal.
Your best bet would be to sit down and draft up a carefully worded, non inflammatory letter, explaining that there are none of these issues (above) and include pics of it all. Then politely request the property manager to pass this on to and seek instruction from the owners of the property and if they still knock it back, then ask under which of the general lease conditions they are telling you to remove it under....remember "polite". Polite will always get you further.
As a landlord, personally I couldn't give a rats arse if a tenant ran a brewery that was well built and didn't cause damage to the property and I think most owners are the same. In fact one of them does. I think it'll come down to the PM not understanding what they are looking at.