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TR6 Front brakes won't release

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John_Mc

John_Mc

Jedi Knight
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Hi all. I haven't been on here in a while so I apologize if this has been recently discussed. My front brakes won't release on my '71 TR6. I have recently replaced the master cylinder and resealed the pressure differential valve, but still when I drive, the situation gets progressively worse until I can't budge. It may be that I need to bleed the lines more thoroughly, but I think there's more to it. Anybody have this same experience and know the most likely culprit and the most predictable solution? Thanks in advance.
 
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Try this simple test: when the problem happens (brakes just dragging, you don't need to wait until it won't move), loosen the fittings on the brake MC briefly and let a few drops of fluid out. Best to hold a rag underneath so the fluid doesn't get on your paint. Tighten the fittings back up, and see if the car will roll freely now.

If that releases the brakes, then the most likely culprit is the pushrod between the servo and the MC. There is some variation from MC to MC, and the pushrod should be adjusted when either one is replaced; so it doesn't hold the MC piston partially depressed (nor have a gap). This operation is unfortunately not covered in the workshop manual, AFAIK.

Best solution is to take it apart, make some careful measurements and adjust the pushrod. But as a short term workaround/proof of concept, you can put flat washers in the joint between the MC and servo.

There is a very little bit more about this in the Buckeye article at https://www.buckeyetriumphs.org/s/Overhauling-Brake-Servo.pdf
(near the bottom of page 8)
 
Also a good possibility that if you still have the rubber brake lines they are starting to collapse internally. They will allow fluid through to pressurize the calipers but not allow it back to release them.
if the above test doesn’t fail, then consider replacing the rubber hoses.
 
+1 to both posts above. In particular to Charlie's.

The flex lines can look to be in perfect condition on the outside... and be collapsed on the inside. The internal collapse turns the flex lines into check valves that allow high pressure brake fluid to reach the wheels... but not release.

I have experienced this on a few old cars including a '69 TR6 earlier this month.
 
If there's pressure at the caliper bleeders when you're brakes are dragging, it's the flex hoses.
 
I replaced the old rubber flex hoses with stainless steel braided hoses (Cobalt brand from Moss) and so far that seems to have resolved the issue. Thanks for all your help.
 
Well, after driving a bit it appears I have not resolved the issue. I'll see if Randall's work-through shows it to be the MC pushrod, but is there anything else I can check? Is there somewhere else in the system that would act as a check-valve? The problem seems bilateral, and it seems to be the front brakes only.
 
Clearly it is not retracting. But why? Malbaby, are you meaning that the piston or caliper cylinder is corroded and causing enough friction to keep it from retracting? I replaced the pistons about 12 years ago, which seems recent to me, but I guess it isn't. If the calipers themselves are corroded/pitted, what are my options? I'm not seeing them for sale through Moss or TRF.
 
Jack up the front end and turn both wheels, that will tell you if one is dragging. That is the one with the cocked piston, it needs new seals and possible new pistons.
Just keep pushing the pedal until it starts to lock up, won't take long.
 
IMHO...M/C pushrod preload as per "Randall"....or caliper piston problem.
If it is the calipers, any dedicated brake shop can reco the calipers for you.
 
John
Moss has caliper on sale 180-568 for the pair.
https://mossmotors.com/catalogsearch/result/?cat=&q=180-568
Or you could use Little British Car Co. lbcarco.com (a Moss Reseller) and save on the Ca sales tax.
And they also discount Moss's price.
180-568
CALIPER,BRAKE,PAIR,L/H,R/H
[SIZE=-1]REDUCED FOR LBCARCO SALE[/SIZE][SIZE=-1]
TR4/4A/250/6, MGC, AHY BJ8(LATE), GT6
Retail $159.99 Moss Sale price $127.99 Our Price $121.59
[/SIZE]
 
Try the other tests before you spend money on new parts.

If you discover you have a stuck piston... by all means try rebuilding the calipers first. You have nothing to lose. If you don't want to try rebuilding or you find that there is damage you cannot overcome, BP Northwest has new calipers for a '71 at about $81 each. Alternatively, if you want OEM calipers that are rebuilt they are available from Apple Hydraulics for $85 each plus a core charge.

If you take the calipers off for a home rebuild, do not freak out if there is pitting in the bore of the caliper casting. The system does not seal on the caliper bore so pitting there may cause the piston to stick but it is not a leak path. On the other hand, the circumference of the seal grooves are important. Pry the seals out then carefully and thoroughly clean and inspect the grooves where the seals go. Very, very minor pitting of the circumference of the seal grooves can be acceptable. However, if in doubt have a pro inspect them or replace them.
 
A late follow up, but it turns out that Randall was correct. It was the MC pushrod adjustment that fixed the problem. Thanks for all your input.
 
uh.....the chances of both front hoses failing the same way at once is slim.
One will cause pulling to one side.
Cracking a caliper bleeder tells you nothing other than there is pressure. Pushrod, master, crushed brake pipe will all hold pressure at caliper. One thing you can eliminate is calipers if there is pressure. They won't hold pressure in the line.
 
I replaced the master cylinder first, because I knew it felt "grindy" when I'd push it, so I suspected it needed replacement anyway. Then I replaced the four rubber flex hoses with stainless steel because those have been on as long as I've had the car (17years) and were probably old back then too. Then I replaced the seals on the PDWA because it was leaking fluid through the switch. When it was still dragging, I bit the bullet and replaced the calipers. At first it seemed all was well, but then the dragging commenced again (bilaterally) so I threw in the towel and took it to a shop and told them to check the pushrod for adjustment. Since then I've had no issues. I wish I would have followed Randall's advice first because he was spot on, but there were so many other components that I knew were questionable, that I really figured I would end up solving the issue when those were done. I could have saved myself buying some new calipers though! :(
 
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