I had a little epiphany the other day about brake fluid. This might be common knowledge to those with more experience, but I never had this knowledge. A little background:

A couple of years ago our noble Treasurer helped me bleed the brakes on my van after getting the master cylinder rebuilt. All worked well but if I was at a stop holding pressure on the pedal, the pedal would slow fade to the floor. A quick pump brought it back up. In the end I ended up bleeding the entire system to rid the issue.

This past August I experienced a similar behavior in my daily driver. I replaced the front calipers, bled the system, and very similar behavior. This has system a vacuum boosted master cylinder.

My epiphany was this: mixing synthetic with non-synthetic brake fluid. To confirm this I used non-synthetic fluid after changing the front calipers on my GF '77 Mercedes. No brake fade.

I do understand there are other considerations: DOT3/4, DOT 5 vs anything else, etc....

It brings up a question, do you prefer synthetic or regular brake fluids? Since synthetic seems to be on every shelf it makes sense to use it out of convenience, however perhaps these older components don't like it? Is synthetic the "ethonol" of brake fluid? Do we need to worry about the seals on the master?