Subject: Re: .75 wheel cylinders
> I'm quite interested in any comments regarding brakes because, in the last
> year, I have replaced front discs & pads, all brake lines, rear cylinders,
> and rebuilt the master cylinder. My brakes certainly don't approach the
> "locking up at a tap" but since I've never had a TR6 before, I don't have
> much to compare to. I believe the brakes are not what they should be and
I
> think the best way to confirm this is to drive someone else's 6.
>
> Rich C.
> Brakes bedamned, I'll be enjoying that dang car this weekend.
Rich,
I've put well over 100K miles on TR6s and have had no brake problems other
than normal wear & maintenance. As I noted earlier, on dry pavement with
full brakes applied, the rear wheels will lock up. I don't think this is
a big problem. On a wet pavement the lockup occurs with much less force.
On snow -- you can imagine. However, this is about the same as a pre ABS
pickup. In spite of my previous sarcastic comments, I think the TR250/TR6
brake system is really pretty good ---- if everything is up to snuff, they
are not that much different from those in a new car with power brakes.
Also, I seem to recall that Triumph was the first to use disk brakes on a
production auto ---- so maybe the TR brakes were high tech for their time.
You didn't mention whether you had someone else do all your brake work or
you did it yourself. The new lines, pads and cylinder rebuilds are to be
expected on ~30 year old cars. Replacing the front rotors (disks)
surprises me since I have over 150K miles on one set and they look as thick
as new, but I haven't measured them.
In my view nearly all the braking is done by the front with the back used
primarily for the park brake. So, if your brakes don't seem up to snuff, I
would look at things that might affect the front brakes.
The first step is to determine what is normal and good. Your thought to
drive another TR is clearly the best. Once in a while when working on my
own or checking a friend's, I/we drive several of mine to get a fresh feel
of good brakes. This is especially important in the spring when we haven't
driven them for a few months. Also, sometimes the systems go bad a little
at a time and we hardly notice. The same is true for the clutch hydraulics.
Some suggestions:
If you have to push the pedal very far before the brakes start to apply ---
like half way or more --- then you should check for air in the lines and
rear brake adjustment. (Note that the pressure applied to both the front
and back is the same, and no significant force is applied to the pads &
shoes until all the pads/shoes are in contact with the rotors/drums and all
air pockets have been compressed to near zero volume.) The rear brake
adjustment is the easiest to test. Pull the park brake on and then push the
brake pedal several times. If the brakes come on (you meet resistance on
the pedal) much earlier on the last push, then the rear brakes need
adjustment. After you do the adjustment if needed and if you still have
to push the pedal too far, bleed all the wheel cylinders.
If the system has plenty of pedal but you think you have to push too hard,
then I'd look at the PDWA and brake servo.
The PDWA has a piston that moves in response to a large difference in
pressure between the front and rear brake lines. Such a difference can be
caused by a leak or air in half the system. This piston moves toward
the low pressure side and tends to cut off the fluid to that side. It's
quite easy to get the piston in the off-center position when installing new
lines and bleeding the system. Normally, the brake warning light should be
on even with the park brake released when the piston is off center.
However, if that area of the brake system has been disturbed as it would be
when installing new lines, then the electrical part may not be working. The
easiest way to tell if the piston is off center is to remove the switch from
the top of the PDWA and shine a light into the hole. If the piston is
centered, there should be a narrow grove around the piston (that the pin of
the switch fits into) centered under the hole for the switch. If this grove
is off center, it should be centered using a sharp object such as a scribe
or awl.
The rubber inside the hoses between the lines and the front calipers
sometimes dissolve and plug these lines. You mentioned new lines so I
assume you replaced these too. If not, the easiest way to test for plugged
lines it to open the caliper bleed nipple and make sure there is significant
fluid flow when the pedal is pressed. This is another way to check for an
off center PDWA piston. You can do this test as part of the cylinder
bleeding mentioned previously.
Another area that can cause poor braking (too much pedal force) is the
brake servo (aka booster). If the vacuum hose from the intake manifold to
the servo is leaking or blocked, then the servo may not be operating
properly. This spring I noted this hose on my TR250, which makes a 180
degree bend at the manifold, looked pinched at the bend. A few weeks later
my brakes acted strange--- sometimes they seemed perfect and later they
required too much pedal force. I then played with the system and found the
brakes perfect if I hadn't used them for a couple minutes but became hard to
push if applied three or more times quickly. On the way home I thought ---
could that line be restricted? It was. There was enough of an opening to
recharge the depression (vacuum) in the servo over several minutes, but not
enough to recharge it immediately. A short piece of 3/8 inch copper tubing
inserted into the hose at the bend serves to keep the hose open ----- it's
an original striped hose that is no longer available so I'll use it as long
as possible.
Other than the above example, I've had no experience with servos that
operate poorly --- they seem to be binary; work or don't work. The test I
use for the servo is to disconnect the servo vacuum hose and then try the
brakes while moving. (The vacuum hose towards the manifold should be
plugged or the engine will barely run at idle.) If the brakes are no worse
with the vacuum hose disconnected, then there is a problem with the
servo system. Someone on the list may have a better test.
Hope this helps, and let us all know what you discover so we can all learn.
Nelson Riedel
Granville, OH
76 TR6 (just painted, now repairing damage from fender bender)
68 TR250 (next project - new paint)
70 TR6 (all the parts in a pile ready to assemble)
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