[How to BENCH BLEED the Master Cylinder. Done when installing
a NEW or REBUILT Master cylinder that doesn't already have
Brake Fluid in it].
Mike,
I had some OLD brake lines that I was tossing out. I just off the
threaded fitting and a few inches of metal line.
I'd bend the lines back so the whole thing looks like a "J".
Put the top of the "J" down into the reservoir, then thread the
fitting into the bottom of the brake master cylinder. Ditto for
both front and rear reservoirs on tandem brakes.
Fill the reservoir with Castrol GT/LMA Brake Fluid (my favorite),
and push in the push rod a few times. You only have to overcome
spring force, so pretty easy to do this when the brake master cylinder
is up on the bench. Keep this up until you do not see any more air
bubbles coming up from the lines.
I use my bench vise, with two thick soft (pine) boards to
protect the aluminum brake cylinder from the jaws of the vise.
Viola... master is now "bench bleed" which makes it FASTER to bleed
the brake system. Disconnect the metal lines, and use an absorbent rag
when go to put the brake master cylinder back into the car. Keep it in
place until everything is bolted up to prevent getting brake fluid
on the car.
Tighten the fittings by hand. Yes, messy with brake fluid. Once you
have the fitting all the way down (keep wiggling it when starting)
then use your 7/16" flare nut wrench to "snug it" home.
TRIVIA:
Brake line fittings are 3/8x 24tpi FINE Thread. Common size for
an american car. Lines are cheap, so you can buy a 12" long line...
cut it (tubing cutter is the proper tool, but a fine tooth hacksaw
blade used slowly will cut it.) Two six inch long "bench bleeding
tools are formed. I used the letter "J" to describe the shape of the
metal line, a "G" or "C" might give you a better idea.
BRAKE MASTER CYLINDER SHIMS: You know those funny shims between the
brake master cylinder and firewall: They are there for a reason!
The original master cylinders used a fixed pushrod length. There
needs to be some free play in the pushrod to master cylinder. Not
much, maybe only 1/32" of an inch. With such a heavy return spring
on the pedal, it is hard to notice the free play. Reach up there, and
you should be able to "wiggle" the brake master cylinder push rod.
OK? My hands are too large to fit, but just so you have the idea.
What happens if there is no free play in the pushrod? It slides back
into the brake master cylinder. As the pistons in the Brake Master
Cylinder slide forward, first thing they do is cover the reservoir
inlet holes. Remember the fluid has to move from and to the reservoir.
So when you cover the return hole, the rear lines will maintain much
higher brake pressure. This will leave the rear wheels dragging, or
even locked up. Adjusting them may seem to solve the problem, but
it returns. So check that pushrod clearance. On roadsters is was
adjusted by shims. On the later 510/240Z Nissan used an adjustable
pushrod. I'd swipe them off those old METRIC master cylinders, and
retrofit them into my Roadster SAE FINE thread master cylinders. Hence
an adjustable pushrod, and no shims to mess with.
Too many words this morning. Sorry for being so verbose.
Cheers,
Tom Walter -no roadster-
Austin, TX -Datsun 510 & Toyota Land Cruisers-
datsunmike wrote:
> I have a book that describes how to bleed a master but I can't remember
> which one and I have many :(
>
> The holes aren't plugged, rather lines are run from them back into the
> reservoirs.
>
> Mike
>
> ----- Original Message -----
> From: "Fergus OFarrell" <FOFarrell@qtww.com>
> To: <datsun-roadsters@autox.team.net>
> Sent: Thursday, August 30, 2001 8:39 PM
> Subject: bellhousing "window"/ bench bleeding (no, not me)
>
>
>
>>Um, folks, I was not so appalled by the picture of the bellhousing window,
>>because my '82 1800cc Subaru had a nicely molded bellhousing window just
>>like that, with the timing marks underneath. (okay, it had smoother edges)
>>You also could use a screwdriver to turn the engine using the flywheel
>>
> teeth
>
>>thru this window. It had a nice little contoured rubber plug for it.
>>
> Okay,
>
>>so maybe the boxer style (flat 4) motor was never capable of
>>
> cafe-spraying,
>
>>flywheel-shattering torque outbursts, but a better water-cooled flat 4
>>
> than
>
>>vw's attempt.
>>Roadster content: my trans tunnel is now completely covered in alum-faced
>>sound deadener, with alum-taped edges; looking "Apollo reflective".
>>When you 'bench bleed' a master/cyl, what do you put in the brake line
>>holes? Corks? Or nothing? Won't it just drain out the bottom?
>>Fergus O, 69 2000 in bits
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