Date:         Tue, 18 Apr 1995 19:19:32 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Alistair Bell <ui775@freenet.victoria.bc.ca>
Subject:      bleeding brakes-a failed gadget


With too much time on my hands at work I made a brake bleeding gadget. The
heart of the system is a 60cc plastic syringe (or "horse fit" as they used
to say in the old days) and a luer 2-way valve. the valve fits on to the
end of the syringe and from on of the 2 remaining ports on the valve a
longish piece of tygon tubing is attached. This goes to the bleeding
nipple. To the other valve port is attached a shorter length of tygon
which ends up in a 1 litre nalgene polypro bottle (via bulkhead fittings
in the bottle cap and a dip tube). 
So I tried it out tonight. I attached the tubing to the nipple, cracked
the nipple, and pulled back on the syringe. Yes, dirty fluid came through
the tubing, and when the syringe was full, I switched the valve so that
pushing the syringe plunger would deliver the 60 cc of mucky fluid to the
bottle. So far so good. I continued this drawing of fluid until I had
collected about 250 ml of fluid from the one (right rear) brake. But I
still had air bubbles coming along the line. So then I rearranged the
tubing so that it went straight from the nipple to the bottle and pumped
the brake pedal. This quickly produced clear, non-aerated fluid from the
brake. I repeated this with all of the brakes.
My conclusion is that when suction is applied to the nipple, it does draw
the fluid out but air sneaks past the nipple threads, from the outside,
into the slave cyl then out the nipple. I could not tell whether the air
coming down the tube was from an exogenous or endogenous source. 
Have i forgotten something, or is this inherent in the "sucking method" of
brake bleeding?
I suspect that using a pressure bleeder would not display this effect, any
slop in the nipple threads would be revealed by fluid oozing out.
So next slow work day project is to modify the brake fluid reservoir cap
by the addition of a bleeder nipple so that I can "pump-it-up" with my
bike pump.

Alistair