Brake fluid question please

[Oldun]

So... 3 out of 4 front/rear brake master cylinders have given up the ghost on my 2 NZ-based BH bikes since last year. This morning the front brake on the 350 locked up at the traffic lights... luckily I got it unstuck (dunno how) and made it to the mechanic (where I was headed anyway for a front tire change). From being full locked on the lever is now moving with no pressure at all, so I assume the master cylinder has gone full fathom five on me. Which leave only the 502's rear brake working (seems I better order a spare now...).

Questions the tech has asked me and which I'd like to ask y'all to chime in on: (1) what brake fluid is best, synthetic or not-synthetic? And (2) given these two are respectively 2006 (the 350) and 2007 (the 502) would they have started off with DOT 4 or DOT 5?

Thanks in advance.

[SRF]

They came with dot 5 fluid, if someone has put dot 3 or 4 fluid in them that would explain your problems because they are not compatible.

[Oldun]

Thanks Shane!!

[BossHossMan]

DOT 5 is a silicone-based brake fluid that does not absorb water and is used in most motorcycles. Look close at your master cylinder. It should have a label that states what brake fluid is required.