There is some confusion in the IVA guide on this. On one hand it says this;
On all fluid reservoirs, it must be possible without opening the reservoir, to check the fluid level relative to the manufacturer's specified minimum level by
• the actual fluid level being visible through a transparent section of the reservoir and with the minimum level permanently marked on or adjacent to this section, or
• a red warning lamp capable of illuminating when the reservoir fluid falls to the minimum level.
So this is saying either a reservoir with a transparent section to indicate fluid level or a red warning lamp will do, but on reasons to fail it says this;
A fluid reservoir
• not having a transparent section through which the fluid level is visible, and a permanent "minimum" level mark, and
• not fitted with a red warning lamp capable of illuminating when the fluid level falls to the 'minimum' safe level.
So there is a contradiction here.
If you were to fit a warning lamp though - and to be honest why wouldn’t you, after all the purpose of the warning lamp is to indicate brake fluid loss which in turn could indicate a leak within the brake hydraulic system which will need investigation - it needs to be a visible red warning light accessible by the driver in all conditions.
It must have a recognised symbol on it such as;
The IVA guide does go onto say with regard a red warning lamp being the only visible indicator of fluid level without opening the reservoir, that it would fail if it was;
• insecure.
• inoperative.
• not visible at all times from the driving position.
• not fitted with an operational "test facility" to check its operation from the
driving position without opening the reservoir.