DeTomaso Mailing List: January 99, Message #266

On Jan 5, 1999, Max Watson wrote;

> A bad master cylinder certainly makes a lot of sense, especially with the
> symptom that the pedal pressure stays about the same all the way down to the
> floor.  Is there any chance I could have damaged my new master cylinder
> during my attempted bleeding (for instance by pushing the brake down too
> far)?  Also, is there any way I go about checking the operation of the
> master cylinder?
>
 Checking the master cylinder is pretty easy.  Use a piece of standard
3/16" steel brake line about 1 foot long, with fittings and flares at
each end, bent into a "U" shape.  Disconnect the car's lines from the
front and rear master ports, and attach the "U" piece to the master's
ports, thereby sealing the ports together.  In this condition, your
pedal should be rock hard and not drop to the floor even with continuous
applied pressure.  If not, the master cylinder is definately the
problem.

I have never experienced, or heard of anyone, damaging a master cylinder
during bleeding.  If a brand new master cylinder is bad, most likely due
to one or both piston seals being installed backwards, torn, or nicked
during assembly at the factory.

Another possibility; what type of brake fluid are you using?  Regular
DOT 3 or "silicone" DOT 5 should be OK.  I used some foreign (German
made DOT 4, I think) fluid many years ago in my van, not knowing any
better.  It dissolved the seals in short order, and my brakes failed
while driving (of course).  Had to use the parking brake to get home.
Nasty.  I had to replace the master, the rear slaves, and rebuild the
front calipers to fix it.

Hope this helps, Rodney.