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Brake problem.


rulloyd
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So every component of my brake system is now new (single circuit GT6).

I have bled them over and over again but if I leave the pedal for a minute it softens until I pump it once.   Then it works well,  but if I leave it a minute it softens again.

Front callipers (new) are working fine. I have pushed the pistons in and out multiple times and they don’t stick on or off at all.

Out of ideas.  Anyone??

All after market parts of course.

many thanks

Rich

 

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Two possibilities:

Theres been cases recently of brake calipers with new seals which arent the same as the originals and they pull the pistons back too far. This means the pedal is soft initially when taking up the extra play but if pushed again is ok because the pistons havent had time to pull back. There should be a few posts on here about this.

The other is a master cylinder with leaky non return valve. Its more likely with old MCs but if the seal that stops fluid returning to the reservoir doesnt seal straight away on pressing the pedal, it will be soft initially. Then pushing the pedal further or pumping will harden it because the seal has now been made. Its difficult to prove this fault although you might see a slight rise in the reservoir fluid level as you first press the pedal where fluid flows back into it instead of building pressure in the system...

 

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Thanks Johny.  Confident it's not the first option as I have checked this already.  I wonder if the Master cylinder is faulty even though it's brand new.  I'll try that test.

Could it be a problem with the servo hydraulics? ( I have a Powerstop servo which I also rebuilt, new seals).   Tempted to bypass to make sure.  No leaks at all.

Thank you

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How I "tested" a master cyl, (It was off the car for refurb/replace) was to fit a brake bleed nipple to the outlet and a small amount of fluid in the Reserviour, Put the whole thing in a vice and tighten hard, then left it for several hours, Because it was still "hard" and did not fall out of the vice. I counted that as a success.

I have had experience On big "yanks" of it taking forever and several "bleed sessions" to get a hard pedal. Hence part reason why I now posses a vaccuum bleed kit!.

Pete.

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a few ideas 

disconnect the handbrake cable any tension will hold the shoes open and foil any atempts to fully adjust especially with self adjusters

get the adjusters wound up before you re adjust  and refit the cable

wheel bearing endfloat   will push pads back when its standing if its excessive 

it may not be bleeding but i have a long tube and the catch bottle up  high above the wheels  to air goes up  strong pedal down nip up slow return

new calipers have a number of posts on here about piston pull back 

do the simples first 

if you get a good squirt on a fast pedal press i would not dream of master or servo cylinder problems 

this has all the hall marks of rear shoe adjustment is in trouble 

 

 

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6 hours ago, rulloyd said:

Thanks Johny.  Confident it's not the first option as I have checked this already.  I wonder if the Master cylinder is faulty even though it's brand new.  I'll try that test.

Could it be a problem with the servo hydraulics? ( I have a Powerstop servo which I also rebuilt, new seals).   Tempted to bypass to make sure.  No leaks at all.

Thank you

Course I forgot about a servo and one could give the symptoms youve described. A test to discount the brakes themselves is to clamp their hoses - if with all 4 closed off theres still a problem it must be with the MC/servo. Then either bypassing the servo or testing the MC as Pete has described (could be done on the car?) would further narrow it down...

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9 hours ago, johny said:

Two possibilities:

Theres been cases recently of brake calipers with new seals which arent the same as the originals and they pull the pistons back too far. This means the pedal is soft initially when taking up the extra play but if pushed again is ok because the pistons havent had time to pull back. There should be a few posts on here about this.

Sorry to be pedantic, but it is not the seals that are the issue, it is the actual calipers. The reproduction calipers are designed wrong, only by what at first glance appears an insignificant detail, but it means the seals grip the pistons too well. 

Nick Jones documented his trials and tribulations somewhere. 

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4 hours ago, johny said:

Course I forgot about a servo and one could give the symptoms youve described. A test to discount the brakes themselves is to clamp their hoses - if with all 4 closed off theres still a problem it must be with the MC/servo. Then either bypassing the servo or testing the MC as Pete has described (could be done on the car?) would further narrow it down...

The flexible pipes are the ss braided variety.  Presume these can’t be clamped?

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11 hours ago, rulloyd said:

Could it be a problem with the servo hydraulics? ( I have a Powerstop servo which I also rebuilt, new seals).   Tempted to bypass to make sure.  No leaks at all.

I also had the soft pedal problem, after rebuilding my Powerstop Servo. I traced the fault to the output union, which was weeping around the copper sealing washer,  but so slightly, it was very hard to see unless you really peered at it. With the union 'nipped up' the fault cleared.

Gav 

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I've had this problem with the soft pedal for years. Pump it and it's solid.  I have recently renewed the rear brakes and slaves and changed the braided lines on the front for rubber. But still after bleeding the pedal is still soft. So I clamped the front brake lines and the pedal is now rock hard. 

So I take it the calipers will need a rebuild. Is there any decent rebuild kits out there. 

Danny

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Im not convinced this will cure the issue unless you have calipers with the problem Clive described previously in which case I presume they need to be replaced. Any brake system will have some softness and other than leaks I cant think of anything to overhaul in the calipers or slave cylinder that will improve it.

Modern cars do have less but of course also have the benefit of modern design and manufacturing so we get to expect the same in our Triumphs. On mine in the end I went to a larger 3/4" master cylinder and perhaps at the loss of a little sensitivity I have gained a more modern feeling pedal....

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the most frequent pad push back is excessive front hub end float 

the other is the handbrake cable holding shoes open so adjustment is compromised 

piston pull back is a new problem with some of the recent calipers on sale , if the calipers are a few years old it wont be this 

always do the simple clues first

when you are up to your arse in Alligators dont   forget you were supposed to be draining the swap !!!!

Pete

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All brake components including pipes and unions are all brand new with the exception of the servo which I have refurbed.  The guy doing the mot who is very triumph experienced first thought was incompatibility between the new callipers and the new (but old style) mc.   He reckons the callipers may be greater capacity.  But I left it with him to look at.

The pistons in the callipers (which are probably 18 months old) are not visually retracting at all but they are working so I don’t think it’s the piston problem.  Nor does the fluid in the mc appear to go up or down when applying the brakes which I assume is right. I did physically push the pistons in and out numerous times and they were moving ok.

I adjusted the rear brakes up tight (no self adjusters).  Made no difference.

I checked the unions on the servo.  All tight.

hub end float good.

thanks for all the suggestions.  

I’ll probably try bypassing the servo.  Then I may try reverting to old callipers.  Or I could try a bigger capacity mc I guess.

Drove it to the mot this morning.  Car is pretty much finished after complete restoration, even replaced chassis.  Had the biggest smile on my face 😁😁😁 (sadly had the rozzers driving behind me all the way😩😩)

 

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Well the original triumph 3/4 ones are very difficult to get hold of and secondhand will probably be knackered. So if youre not too worried about originality ebay has plenty of them (they are used on early Land Rover clutches) and its probably best to go for one without a reservoir so that you can fit a larger plastic screw on one...

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1 hour ago, rulloyd said:

I adjusted the rear brakes up tight

question  did you disconnect the handbrake cable  when you locked them up 

sorry to repeat but any cable tension holds the shoes open and youre making shoe contact but not bottoming the shoe on the wheel cylinder 

so there is a gap and your MCyl drops as the  wheel cyl closes the gap to the shoes    hence 2nd pump gives a firm pedal 

walk away the gap returns and down goes the 1st pedal pump 

Pete

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3 hours ago, Pete Lewis said:

question  did you disconnect the handbrake cable  when you locked them up 

sorry to repeat but any cable tension holds the shoes open and youre making shoe contact but not bottoming the shoe on the wheel cylinder 

so there is a gap and your MCyl drops as the  wheel cyl closes the gap to the shoes    hence 2nd pump gives a firm pedal 

walk away the gap returns and down goes the 1st pedal pump 

Pete

Hi Pete.  Sorry you had to repeat yourself.  You assumed correctly that I did not disconnect the handbrake.  It’s the 1st thing I do when I get the car back 👍

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