I had the clutch slave replaced on my 69 S2 a year ago, so the fluid is new. The shop where I used to go closed, so I don't have a clue as to what fluid was used. I know it wasn't silicone based. The fluid in the reservoir is very clear. Does it matter if I top it up with DOT3 or DOT4? I noticed that the fluid bottles now say 'synthetic'.. is mixing a glycol base with a synthetic OK?? or is all Clutch/Brake fluid now 'synthetic'?

 

If you're unsure about the current contents of the hydraulic clutch system, change the fluid and replace with DOT3, which is less hygroscopic.

 

Interesting, I didn't know what the difference is. It figures, though, because the clutch fluid is never going to get fluid in the brake lines at the caliper end..

 

From the Valvoline website:

WHAT DOES BRAKE FLUID CONSIST OF?

Brake fluid consists of solvent (between 60 and 90%), lubricating agents (5-30%), and additives (2-5%). Furthermore, brake fluid additives include corrosion inhibitors, which prevent corrosion of the brake system metal parts, and antioxidants, which prevent solvent breakdown and deposit formation.

GLYCOL BASED

Brake fluid DOT 3 uses glycol ether as a solvent and a conventional brake fluid used in vehicles produced up until the 1990s. It is suitable for all brake systems and all driving conditions.

Brake fluid DOT 4 uses glycol ether and borate ester compounds which improve its performance compared to DOT 3. Cars produced after 2006 use it as a standard. Like its predecessor, it is suitable for all brake systems and driving conditions.

Brake fluid DOT 5.1 uses glycol ether and borate ester compounds but has an enhanced performance like the silicon-based DOT 5. We can say that it is a non-silicone version of DOT 5 but compatible with DOT 3 and DOT 4 types of braking systems and fluids.

All glycol-based brake fluids need to be handled carefully because they can damage the paintwork of the vehicle or motorcycle.

SILICONE BASED

Brake fluid DOT 5 is a silicone-based fluid making it incompatible with anti-lock braking systems. Additionally, DOT 5 cannot be mixed with other brake fluid types and unlike other fluids, it does not damage the paintwork.

This fluid is engineered for demanding applications like military vehicles and extremely cold climates.

CAN WE MIX BRAKE FLUIDS?

Yes, but with certain exceptions.

Brake fluids can be mixed only if the fluid that you add to the system belongs to a higher grade.

For example, a braking system that has a brake fluid DOT 3 specification can use a DOT 4 or a DOT 5.1 fluid. Also, a system with a DOT 4 can be topped up with a DOT 5.1 without bleeding the system.

However, never top up a DOT 4 system with a DOT 3 brake fluid. DOT 3 won’t be able to meet the temperature requirements because of its lower dry and wet boiling points.

Once again, do not mix DOT 5 with any other type of brake fluid because of its silicone-based composition.

For optimal braking system and brake fluid performance, it is best to bleed the brake fluid and replace it with a fresh one.

 

Wow!!

That is the most comprehensive treatise I have ever experienced on brake fluid. Well Done.

 

mbwillgrubs... can you post the link to the Valvoline site? I would like to read more about the fluids. Thank you!

 

Sure, Valvoline Fluids URL

 

Thank you for all the great info. The resolution/solution was pint of Valvoline's combination ( my word) DOT 3 4 fluid.

https://valvolinechemicals.com/produ...ake-fluid-32oz

A bit of work to find the smaller size, a local NAPA had it.