Post by SH Post by newshound Post by Michael Chare
What tools would a garage (main dealer) use to change the brake fluid
in a car? Do they have something that attaches to the resevoir and
supplies the new fluid at pressure so that the operator can let the
old fluid out of each caliper in turn?
I am only guessing, but I have always assumed they used the
professional equivalent of the old "DIY" Eezibleed which is just as
you described. A bottle for new fluid, connected to a replacement
filler cap of the reservoir, and pressurised with air to 5 or 10 psi.
Then each brake nipple opened in turn until the fluid runs "clear".
Usually starting with the brake furthest from the master cylinder.
I have seen vacuum versions where the fluid is sucked out under a vacuum
from an opened bleed nipple on the calliper/psiton.
The brake fluid reservoir screw cap is replaced with one with a tube
connected to it which then goes into a container of fresh brake fluid.
Vacuum is preferred over pressure as the effects of a leak is less....
i.e. the system would suck in air via a leak whereas in a pressurised
system, hydraulic fluid would be sprayed everywhere!
Well not really sprayed everywhere, if (as was common) you connected
your easybleed to a tyre to provide the pressure, it was recommended
that you dropped the pressure to 1 bar or so, because plastic master
cylinder reservoirs are not designed to take pressure! Yes, it would be
messy if you "blew up" a master cylinder, but the flow rate at the
nipple or any other normal leak site is not very large.
I've never seen a vacuum system, but no reason why it should not work.
Except of course that every seal in the system is designed for positive
pressure, so negative pressure has the potential to draw air in at each
seal, possibly with dust contamination. With "healthy" seals there
should be enough pressure from the elasticity to make sure this does not