GAZ24.ru meint zu BSK, DOT3;DOT etc:
Before 1985, hydraulic braking systems and clutch linkages of all Volga cars were filled at the factory with Castor oil / butyl alcohol based crimson brake fluid named BSK (DOT-2 analog). It is not interchangeable with, nor can be mixed with, any other types of brake fluids.
This type of brake fluid was developed in mid-1950s and used in USSR/Russia well into 1980s and even 1990s. It was actually good for what it was designed for, namely – braking systems with drum brakes and a low level of heat generation. It is no longer in production because it cannot be used in cars with disk brakes due to its relatively low boiling temperature, and in fact it is more expensive to produce than modern synthetic break fluid (some manufacturers produce BSK “analogs”, but these products are in fact cheap glycol based brake fluids and are not compatible with the original Castor oil-based BSK, or the type of rubber originally used in the Volga brakes; don’t use this surrogate stuff).
Another problem is that – if enough alcohol, which is used in this type of break fluid mostly as a preservative, escapes the system through evaporation, the castor oil would eventually rot and turn into disgusting jelly, ceasing to function as a hydraulic fluid and clogging up the braking system completely. This usually happens if the car was unused for a long time and fresh break fluid or alcohol were not added to compensate for evaporation. The only proper course of action in this case is a compete overhaul of the entire system.
Warning: the type of rubber that was originally used for the hydraulic seals in these cars is not compatible with glycol based brake fluids.
DO NOT use any other type of brake fluid unless all and every rubber part in the system is replaced with a brand new one ! DO NOT mix Castor oil based and Glycol based brake fluids ! In both cases the hydraulic braking system would be rendered inoperative (either instantly or in some time) because the rubber would turn into goo.
Mineral oil based brake fluids are also quite lethal for the rubber used in these systems.
If the level of fluid in the brake or clutch master cylinder reservoir filled with crimson brake fluid drops, as a “roadside fix” you can top it up with pure castor oil (available via drugstores / pharmacy stores), that would lower its boiling point a bit, but the brakes will continue functioning properly. Butyl alcohol (butanol) is also good for this purpose (but somewhat harder to get). Ethyl alcohol (ethanol) can be used as well, but it evaporates relatively quickly, causing the level of the fluid to drop, and also has a lower boiling temperature than butanol.
The applicability of silicone-based brake fluid (DOT-5) is still in question, some owners have experienced compatibility issues with such fluids.
GAZ-3102 (front disk brakes), GAZ-24-10 (either disk or drum brakes) and other later Volga models use standard Glycol-based DOT-3 & 4 compatible brake fluid.
Note: some of the cars may already have been converted to DOT-3/4 brake fluid – check the brake master cylinder and clutch actuator reservoirs for what type of fluid they are filled up with. Castor oil based brake fluid is crimson to orange and has a very specific odor; DOT-3/4 is transparent or yellowish (tends to become black over time because of tiny particles of rubber and metal dust floating in it) and almost has no smell. BSK does not mix with water (they will separate into layers), while DOT 3/4 will mix with water in any proportion. It is not uncommon that even in the same car the clutch linkage and the hydraulic braking system are filled with different types of fluid.
While rebuilding the brakes, always use castor oil / glycol based brake cylinder assembly paste (Rubber Grease) to lubricate cylinder bores, pistons and all rubber seals.