If the market shoots up, you gain equity and rent growth, you can cash out refinance to buy more or save up the extra cash flow to invest when the market softens.

The water softener system seems cost prohibitive at this time.

The market is fairly high right now (not just CA, but most other RE markets too) and frankly I'd be amazed if we don't see a correction or at least a softening within the next couple of years.

If you have 10 properties that cash flow 300 a month(after all expenses)  If suddenly 4 of those go vacant for 3 months, that hurts, but you have 6 other properties that are providing cash flow to help soften the blow.

Good evening,I've been a Boston-area landlord over several years now, and in the past have never had any issues finding tenants all too eager to pay rents that have invariably increased from the preceding lease year. ...

He's a soften spoken gentleman, but don't let that fool you.

A few drought-resistant plants near the back door will help, too.If budget allows, invest in as many hardy trees that will tolerate the dry climate, grow fast and upward against the back fence, to soften the huge two-story apartment staring down at you.

If we had too much inventory or other factors that showed that things will soften in the soon future I'd say wait, but nothing indicates that will happen and rental occupancy is at 99% currently with only luxury being built so it is and appears to remain a landlord's market in my opinion. 

A little softening, yes but as @William Anderson's super thorough post says "seems as of the air is being let out of the balloon at a very slow pace." 

Costs are estimated to sell within 8 months or less with an alternate plan to rent or lease-to-own if market softens.