I'm not sure on TR7's but on some of the other cars I've owned (Ford,
Chrysler, mostly) with dual brake circuits, bleeding the brakes by pumping
the pedal was not always effective.
The distribution/proportioning valve sometimes senses the air as a leak and
moves to shut off that circuit. You can pump aggressively and get some
fluid to move, but the valve does what it's designed to do and prevents you
from properly bleeding the circuit. Hence, the slow pump gets you nothing,
the fast and furious pumping gets you something. Also, the fast and furious
method sometimes induces yet more air into the master cylinder on the
backstroke. And the same thing could potential happen if a wheel cylinder
retracts too quickly. That's one of the reasons that braking systems are
often designed to maintain 2-10 lbs/in2 of pressure in the circuit. It's
not enough to activate the brakes, but high enough to keep out air.
On circuits like this, I've had the most success with vacuum bleeding. I
use a MityVac hand held pump to pull the fluid from the wheel cylinders
rather than trying to push it from the master cylinder. I try to start with
the wheel which is the farthest away from the master (on LHD cars that's
usually the right rear) and work my way forward.
I've never had a M/C give up during the bleeding process, but stranger
things have happened.
> -----Original Message-----
> From: Allen Nugent [SMTP:A.Nugent@unsw.edu.au]
> Sent: 16 July 1998 09:56
> To: triumphs@autox.team.net
> Subject: Bloody brakes (long and pitiful)
>
>
> Dear Listers,
>
> Over the past few weeks I have:
> 1 - rebuilt calipers
> 2 - turned discs
> 3 - replaced rear flex hose
> 4 - removed pressure-reducing valve
> 5 - checked master cylinder function
> 6 - attempted to check pressure-reducing valve function
> 7 - removed and un-blocked steel line from pressure-reducing valve to rear
> circuit
> 8 - replaced steel line from rear flex-hose to right rear wheel cylinder
> (twice)
> 9 - replaced steel line from right rear wheel cylinder to left rear wheel
> cylinder (twice)
> 10- replaced right rear wheel cylinder
>
> Items 8 & 9 were initially replaced with 2nd-hand parts, from a parts-TR7
> in
> storage. When the unions still leaked, I blocked off the rear circuit
> (with
> a bleeder screw in the outlet port of the right-rear wheel cylinder) and
> drove to a brake shop. He found the unions stripped, and the flared tube
> ends incapable of sealing. Moral: don't buy used brake lines!
>
> Now, the car much worse than it did with a leaking caliper and a blocked
> rear circuit; the feels mushy, and has excessive travel. I guess the
> master
> cylinder had seen all it's mates getting rebuilt or replaced, and decided
> to
> join the fun.
>
> My mate and I were pretty aggressive when we tried to bleed the rear
> circuit. He would pump ten times, hold it down hard, then I'd open bleeder
> for a few seconds, and air would gush out. Being gentle achived no
> movement
> of air or fluid whatsoever. (I also had an Eezy-Bleed supplying about 20
> PSI
> to the master, during the operation.)
>
> So, has anyone known a master cylinder to pack it in after a heavy
> bleeding
> session? (It is a rebuilt, sleeved unit with about 4 years' service, and
> about 4 years of disuse.) Should a TR7 be _this_ difficult to bleed? (NB:
> The fronts were a piece of cake.)
>
> To say that I'm reaching the limit of my patience would be like saying
> H-bombs are loud. I must have run over Henry Ford's granny in a former
> life!
>
> Allen Nugent
> Graduate School of Biomedical Engineering
> University of New South Wales
> Sydney 2052 Australia
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