Brake light just started showing up on the dashboard. First item I will be checking is the Master Fluid Level . . . the manual says DOT3 or DOT4 brake fluid . . . How do I determine which one is already in the cylinder ? I can only hope the techs in the past (when they had to replace) used DOT4 due to the higher boiling point, but have no way of verifying. Please advise - thanks.
Dot 4 supercedes Dot 3.
If you put Dot 4 in, no problem. Basically, Dot 4 can go in and replace Dot 3, chances are you already have Dot 3 in anyway.
Wikipedia, Dot 4:
While a vehicle that uses DOT 3 may also use DOT 4 or 5.1
Note: Dot 5 is not compatible with either, it is a different chemical makeup based on silcon. Putting Dot 5 in a Dot 3/4 system is bad.
A couple of things here. First, when I find the brake reservoir low, I will look at my disk brakes to ensure the pads aren't shot (or nearly worn out). Usually, if worn enough, the fluid level will drop to a point where the brake light will come on as you described (most notably so if both back and front are worn out).
With that in mind, consider just flushing your fluid completely using the fluid of your choice (DOT3/4/5.1) and you'll be fine. 5.1 is a bit harder to find, but can be had on the internet fairly easy. Most people do not flush the fluid in their brake system often enough anyway (should be done around a 2-3 year interval on most vehicles).
Even if you just added clean/new fluid to the reservoir to top it off to get you by, you'd be fine. If the brake fluid in the system is DOT4 and you put DOT3 in, not only is it compatible, but will work together. You'd just consider the brake fluid to perform at the DOT3 level instead of the DOT4 level of performance.
As far as DOT5, the main reason you can not use it is because of anti-lock brakes (ABS). The ABS will not function correctly with the DOT5 silicon based brake fluid. It is used by the US Military in their vehicles and in vehicles which are used exclusively in extremely cold climates.
One last thing to consider. If the fluid is low and the brakes are in good shape, keep an eye on it, as you are probably losing fluid somewhere. Leaking brake fluid can sometimes be hard to spot, and usually only happens during braking, so you wouldn't necessarily see it as a spot in the driveway. Just something to consider.
Most Dot 4 Fluid is dyed Blue where as Dot 3 is virtually clear if you cannot determine the colour then it is time for a system flush if the manufacturer says either is acceptable then go with Dot 4. PS Toyota recommend Dot 3 For most of their vehicles and this recommendation must be adhered to as Dot 4 Fluid will damage (soften & swell) genuine Toyota cups and seals.