So, I'm working on a '95 XJ6 4.0 USA version. I was having trouble with the front brake calipers. Replaced both with rebuilt units. Having a bad time bleeding even using the suggested looping the bleeder hose 6 inches vertically and following instructions verbatum. I opened the nipples to try the drip method. Went to check it just now only ti see the barely legible writing on the resivoir reading DOT 4. I filled it with DOT 3. Now what? Any suggestions before I find a bridge to jump from?

 

Hi italcarnut,

Don't be too hasty to find a bridge. DOT 4 has Borate Ester added to the base Glycol Ethers to give it a higher boiling point than DOT 3, but as far as I have been able to determine, the fluids are compatible. The DOT 3 should not harm the seals in your brake system.

The dry boiling point of DOT 3 is 205C vs 230C for DOT 4, and the wet boiling point of DOT 3 is 140C vs 155C for DOT 4. Not huge differences, especially if you are a non-aggressive driver who isn't likely to overheat the brake fluid.

However, some references state that Anti-Lock Braking modulators in vehicles for which DOT 4 is specified may not operate properly on DOT 3 due to a difference in viscosity, so if you notice any odd ABS behavior it would be prudent to replace the DOT 3 with DOT 4. This can be done relatively quickly with an inexpensive hand vacuum pump or pressure-assisted bleeding system.

Cheers,

Don