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View Full Version : TR2/3/3A TR3 Brake Rotor Thickness



lazybird
04-08-2012, 03:24 PM
Anyone out there know the minimum allowable thickness of the 11" diameter front brake rotor used on a TR3?

Ted Enderle

CJD
04-09-2012, 07:52 AM
Hey Ted,

I can't find it either. The manual does say not to true by removing more than .060". So now you need to find what the new thickness was. Maybe there is a portion on the disc that wasn't worn to take a measurement...?

John

TR3driver
04-09-2012, 08:07 AM
Original thickness was very close to .500", so presumably the factory meant that .440" is minimum.

That said, I've seen people run them down to half that, with no apparent problems. Those rotors are really big and well cooled compared to the car's weight; it's tough to overheat them (except perhaps on the race track).

DNK
04-09-2012, 10:45 AM
Ted, welcome.
You find Randall has and all have the answer to anything you need and things you don't.

"nother Washingtonian, welcome again

lazybird
04-09-2012, 11:20 AM
Thanks to all respondents. I couldn't find it in any of the literature. Here’s the deal: on the road or in the garage, initial brake pedal travel is long, second pump comes up solid. Next few pumps always solid. Wait awhile, long pedal again. M/C pushrod is adjusted for minimum play. In the past few years, I’ve swapped Girling master cylinders (both .7” and .75”), rebuilt calipers at least twice, tried both .75” and .7” Girling rear cylinders, alternated DOT4 and DOT5 fluids (flushed each time), used the OEM Residual Pressure Valve, no RPV, Wilwood 2 psi RPV, Wilwood 10 psi RPV. Have always pressure bled. Replaced pads and shoes, replaced hoses (AN-3 braided stainless/Teflon). Brake lines from m/c to each wheel are 3/16”, 1/4” from reservoir to m/c. No leaks. Strangest thing - if I plug the line to the rears and bleed, leaving front calipers enabled, pedal is always solid. Likewise, if I plug the hoses to front calipers and bleed, leaving rears enabled, pedal is again solid. It’s as if the m/c piston can’t move adequate fluid to the four wheel cylinders on the first stroke. If air is being drawn in, I can’t find it. Thought that my skimmed OEM rotors may be too thin allowing excessive caliper piston travel. I'm grasping at straws here, admittedly. Pressing on, regardless…

Apology for the long post.

Ted

TR3driver
04-09-2012, 12:52 PM
Wow, that is strange. Does the pedal go down even if you are not driving the car?
Have you checked that the front hubs & rotors run true? No thickness variation in the rotors. (Yeah, I know you had them machined. Machine shops sometimes make mistakes.)

lazybird
04-09-2012, 02:00 PM
Much thanks for the suggestions, Randall. I’ll check rotor runout with a dial indicator. I check the front wheel bearing play every spring, no problem there. For awhile I thought it might be the proverbial “spindle flex knocking the pads away from rotor”. But the first pedal press is always long whilst driving, parked, sitting in the garage, whenever. I haven’t driven the car for a week – just went out, sat in it, pressed brake pedal – yep, long travel. Second press - short travel, firm feel. I should add that the first pedal press doesn’t go to the floor, the brakes do finally apply close to the limit of travel. Perhaps the fluid from the reservoir isn’t filling the m/c fast enough, until the port closes. But properly adjusted brakes require little fluid displacement. The front pads sit very close to the rotor; I expand the rear shoes until they contact the drum, then backed off a click. I’ve asked my assistant (wife) to apply the brakes while I inspect piston movement at each wheel. They all move just fine. Would rather avoid a larger diameter m/c - too expensive. I still have the front calipers that came with the car, the earlier type with an external crossover pipe. I replaced them with the correct internal crossover type around 1987 (commission # is in the low 70000s). Thinking of putting them back on for a test. I remember that the brakes worked as designed when I lived in Pennsylvania, years ago. Seem to have gotten weird when I moved to greater Seattle in the early 90s. Maybe it’s the climate!

DNK
04-09-2012, 02:18 PM
Any chance the M/C is leaking air into the system

lazybird
04-09-2012, 04:27 PM
It is possible that air is sneaking past the piston seal. I've experimented with three different Girling master cylinders and five rebuild kits of varying age, vendor and manufacturer, with no permanent fix. I've even replaced the brake M/C with a working clutch M/C, which improved the pedal response a bit. Didn't give me 100% confidence in the braking system, however. As noted previously on the forum, the size and composition of piston seals vary greatly between manufacturers. Pegasus Auto Racing Supplies in Wisconsin (www.pegasusautoracing.com (https://www.pegasusautoracing.com)) offers Girling M/Cs and rebuild kits that I've found to be of consistently better quality than those available from the TR parts vendors. I plan to keep driving and experimenting. Beats yard work any day.

Ted

DNK
04-09-2012, 04:56 PM
Were you at LB?

lazybird
04-10-2012, 04:24 PM
Probably not! What's LB?

Ted

tinman58
04-10-2012, 06:00 PM
Lazy Bird?

DNK
04-10-2012, 06:05 PM
Bingo,Dan

And that was suppose to be
" Where are you at,LB?"

tinman58
04-10-2012, 06:15 PM
Funny after reading your post my TR3 does the same thing. I have done just about everything that you have, with the same results. I am just living with it. Not a big deal (unless you have to stop real fast on the first push)

Alec
04-10-2012, 07:09 PM
If you maintain pedal pressure for say two minutes on the second try (when it goes firm) does the pedal still stay firm or does it start to sink slowly? I have had the situation where the seals would operate poorly on first pedal application and then "apparently" seal on second pump only to lose it slowly. This was due to very small pieces of crud (glue) in my case from the m/c re-sleeve process.

lazybird
04-10-2012, 08:38 PM
I'm used to the double-pumping, but every so often I put the car up on jackstands and resume the search for a fix. The pedal stays firm on the 2nd pump no matter how long pressure is applied. In fact, I've pumped it vigorously 10 times; it is firm every time. Everything one could desire in a brake system, except for the first application!

Don - I'm on Bainbridge Island. I let my Tyee membership lapse after the 2001 Breckenridge VTR national.

Ted

TR3driver
04-10-2012, 09:11 PM
I didn't mention this before; but since it seems to be more common than I thought:

Have you checked

1) The rear brake slaves always return immediately so the piston bottoms in the cylinder? Weak return springs (possibly combined with stiff seals and/or rough bores) might cause them to hang for some time after the brakes are applied.

Hmm, come to think of it, the soft line to the rear axle could do the same thing. In my case it held the brakes on hard enough that they overheated, but it might only make them slow to return.

2) Check for any possible binding of the front pads where they sit against the calipers and anti-rattle springs. A notch or groove could be causing the pad to flex slightly when the brakes are applied, and then slowly straighten out over time.

Not really relevant, but many years ago I solved a somewhat similar problem on a Peugeot 504. It had pins that had to slide in the caliper when the brakes were applied, and the pins would seize in the holes due to rust. Sounds trivial, but it had stumped many professional mechanics (and was apparently the reason the previous owner had traded it in). The cure was just to ream the holes out a bit larger, and coat the pins with brake grease.

CJD
04-11-2012, 08:15 AM
And the obvious...have you adjusted the rear brakes? If they are way out of adjustment the symptoms would be identical to what you describe.

John

lazybird
04-11-2012, 09:38 PM
Thanks to all for the suggestions. I've always adjusted the rear shoes outward until the drum won't turn, then back off a click or two. I do like the front pads hanging up on the caliper idea, though. Will investigate this weekend.

Ted