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Sticking Brakes


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Can anyone help with a sticking brake problem on my 1965 TR4A? I realise that this issue has been discussed before and I have re-visited previous comments but none of them seem to quite fit.

 

The problem is not persistent but every now and then after applying the brakes they stick and hold the car back when trying to accelerate. The brake pedal goes rock hard but pressing it firmly releases the brakes and I can continue a while longer. I have a servo fitted and previous responses have suggested that this could be the cause of many braking problems - purists advise ditching it and resorting to a firm right foot but I would like to retain it unless it becomes a last resort. When restoration was completed just over a year ago the whole system was drained, the master cylinder was replaced and refilled with dot 4 fluid. There doesn't seem to be any obvious problem at the wheel end.

 

I am hoping that some of you regular TR gurus can suggest a solution.

 

Thanks

 

Bob

 

PS Would it be harmful to disconnect the servo pipe at the manifold, block the hole and try using the car without assistance before replacing it?

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Two suggestions

 

Firstly relating to the servo there should be a small gap between the servo output rod and the master cylinder input, is this isnt present it could prevent the brakes releasing, perhaps only when hot.

 

Secondly do you have a tipping valve in your master cylinder? If so a faulty/jammed one can give the same problem.

 

Hth

 

Steve

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Hi Bob,

have you any idea where the problem may lay. Otherwise you will need to do some diagnosis.

 

The rear shoes should slip on the slave cylinder in order to centre. The slave can get stuck - this may hold the brakes on.

Rubber hoses at all four brakes can disintegrate internally and hold pressure

The servo can do all sorts of things.

 

Isolating the servo is a good first step.

Clamp off rubber hoses individually to see what happens.

 

Roger

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The air valve on the servo is faulty...this may be combined with a hydraulic seal failure or corrosion inside the servo.

 

You can prove this by temporarily blocking the vacuum hose to the servo & the problem will go away

 

probably easier to replace it rather than try to fix it

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If its an internal fluid fault then just disconnecting the vacuum wont make any difference.By the sound of it the tipping valve is sticking so the only way to tell would be to take the servo out of the system altogether, I have had a few failures with the cheap servos a couple of years ago and that was put down to a bad batch.

Stuart.

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Two suggestions

Firstly relating to the servo there should be a small gap between the servo output rod and the master cylinder input, is this isnt present it could prevent the brakes releasing, perhaps only when hot.

Secondly do you have a tipping valve in your master cylinder? If so a faulty/jammed one can give the same problem.

Hth

Steve

As it's a 4A it'll be a remote servo so no actuating rod. I'm not sure there's a tipping valve either in a 4/4A system. Edited by peejay4A
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Thank you all - some good points here to prompt a more thorough investigation.

 

Unfortunately work will be delayed for a week as we will be away. I'll also do some research on tipping valves as I am totally unfamiliar with either it's function or location. If I manage to solve the problem I will post again for the benefit of anyone else experiencing a similar problem.

 

This forum is brilliant - unlimited expertise.

 

Thanks

 

Bob

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As it's a 4A it'll be a remote servo so no actuating rod. I'm not sure there's a tipping valve either in a 4/4A system.

There isnt.

Stuart.

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Generally in these types of things, the best thing to do is work through a logical sequence eliminating various possibilities one by one.

 

One place to start is to confirm that it is or isn't the whole system, or is it just front/back or any specific wheel? That might be tricky with an intermittant fault which only turns up when your driving (tough to stop and jack up various wheels to see what is sicking and what isn't) - but if you CAN stop and see...

 

If it was an earlier car, I'd suggest looking at the restrictor valve in the three-way junction, but I'm not sure whether 4A's have them.

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Bob said " The brake pedal goes rock hard but pressing it firmly releases the brakes and I can continue a while longer." this is the classic failure mode on an e-type jag or XJ6, it's caused by a seal failing in the servo causing the high-pressure 'downstream' fluid to leak back to the upstream 'foot pressure' side...that causes rock solid pedal and the servo airvalve to be open...some form of servo death spiral...pressing the brake hard temporarily fixes the seal failure and normal service resumes... => replace or rebuild servo

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Bob, if it is a collapsed flexible hose it's unlikely that 2 hoses have failed , so jack up rear of car, with engine running to activate servo apply brakes and get someone to check if just one rear wheel sticks. Repeat for fronts and if it's only 1 or 2 wheels sticking then it's a hose and not the servo.

Chris

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Thanks again everyone - lots to consider when I get home next week.

 

I must admit that having read earlier stuff I was resigned to replacing the servo as Dave suggested but at least I can look at some cheap options first.

 

Bob

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