I can see a mechanics initial reservations as many of us grasp the DOT5 as being the fully synthetic brake fluid. Placing DOT5 in a DOT 3 or 4 system will eat seals.

However Motul indicates the RBF600 is indeed a glycol based (DOT3/4) synthetic, and is therefore compatible with existing DOT3/4 systems.
Sorry, but your mechanic probably did not understand your request or the product itself.

Vince

 

To go deeper, you will not feel mushiness from the presence of water. Water cannot be compressed, so in its natural state, brakes will feel just fine.
The problems happen when the water molecules heat enough to turn into vapor (steam bubbles). Now you have a problem, because the bubbles can and do compress, giving you a mushy peddle.
Old fluid has a larger percent of water, and when you stress the brakes (towing, downhill drives, aggressive driving) you come to a point that the water will flash off into steam quickly, and poof, your brakes are gone.

The older the fluid, the more water is has.
The more water it has, the lower the boiling point.
The lower it boils the sooner it will happen simple daily driving
When it boils, yer done...

If you never get your brakes hot, then the 100% fluid state is in tack, (giving you a faults sense of security) allowing for a solid feel, but the high water concentration is also eating metals (corrosion) on all your internal components including the ABS module.

 

Brakes: The one item on your car you should never short budget wise. Failing to proceed is mostly inconvenient. Failing to stop... <ominous orchestra music>

 

I thought so! But he was insistent so I second guessed myself. Thanks for the clarification.

 

Just want to add I find the brakes on my 2013 XKR bite quickly and stop me like no other car I owned. When I test drove a GT-R a few days ago I found the brakes required a lot of pressure to stop. For street driving I much prefer my brakes on the XKR and want to keep them sharp. I never fully changed the brake fluid in any car I previously owned. I only added brake fluid when the calibers where worked on etc. I don't think the average person even considers changing the brake fluid and when they require to have their calipers changed I'm sure they only add the required fluid and do not do a full flush.


After reading this thread I'll be changing my fluid every 2-3 years, full flush. The next owner will appreciated this and should keep the brakes in top condition and prevent internal rusting.

 

ROTF! But I do agree with you.