Our MOT garage, not a Mazda dealer I should mention here, exchanged the brake fluid early 2018 from the original DOT3 to DOT4, without asking us.
Almost instantly we had problems. The front disc brakes did not return after braking.
The brake pads kept in place against the discs and when driving corners you heard the pads humming.
This is caused by the additives in the Dot4 brake fluid. This fluid is much more aggressive than DOT3 fluid. No problem for modern cars. But our 2004 Mazda could not cope with DOT4. The rubbers swoll and got thicker. Clearly to be seen at the filler cap. Hardly impossible after a month of DOT4 to get the rubber back in.
So, although trusted websites and garages, so_alled experts advice to upgrade to DOT4, Never Ever do this. There is a reason for the message ONLY DOT3.
I have exchanged the fluid with DOT3 now, and I hope that this solves our problems. Fingers crossed.. Otherwise a complete revision of brakes and clutch is required... 4 brake calipers, 1 master brake cylinder, 1 master clutch and the clutch servo... Not the costs, but a lot of work. Although, after this any DOT will be possible.. Hmmm. Maybe not that bad a thought after all.
If you really need to go from DOT3 to DOT4 without changing cups and rubbers, do the simple test as described below:
Take the rubber from the filler cap out, put it in a closed jar filled with fresh DOT4 for a week and fit it back in.
If it fits perfectly, go ahead with DOT4.
And the garage? They are the expert, no way they will repair our car... In their opinion the car is too old anyway and it should be EOL.