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Soft brake pedal in spite of much bleeding


Dave McHattie
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I am currently restoring the complete brake system on my Spitfire Mk 4  Complete system is new including all pipework, cylinders, shoes/pads and calipers/pads and filled with DOT 4 new brake fluid. Having bled the system until no air bubbles seen, the pedal goes almost full stroke until pressure is felt, have subsequently bled with a pressure system  with no improvement, then after leaving it for a week or so, bled with a vacuum system and  had a few bubbles, but no improvement in the stroke. I have of course checked the movement of all shoes and pads and they are fine. the master cylinder is GMC224 which I think is 0.75" bore (5/8" corrected). I must be missing something obvious, can anyone help?

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how did you set the rear shoes ??   cable off ,lock up hard the adjuster , refit cable to just fit , back adjuster 4 clicks 

excessive front hub endfloat will give extended foot travel.

is the master cyl pushrod fully returning so the rod does give a small end float /gap to its piston foot off. if this is restricted the recuperation will not refill the cyl .

Pete

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1 hour ago, Dave McHattie said:

Wow! thanks all for your great support....I'll try out your idea first Tony as it's easy and may quickly identify a problem area, then progress to your ideas Pete.

Cheers Dave 

Sorry Dave, i should have said include Pete's answer with mine. My suggestion will identify where the problem is? And Pete's will fix it :) (i hope)

Tony. 

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i only say 4 clicks , one full turn from what it takes from experience of  hard locked to free enough to turn the drum with a wheel on

its only a guide . may take a few more or a few less ,  you cant fir the leading shoe wrong but  i have often found  the  trailing is  inverted 

Pete

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Last ones I adjusted were the new shoes on the GT6; I locked them as fully as they would go with a square-holed spanner specially for the adjuster, then slacked off one click and they rotated freely. Two clicks was way too loose. The reason I was asking is because it's an eccentric adjuster, and doesn't necessarily slacken off in exact increments, so I was wondering if this was maybe an old trick to bring one particular side of the adjuster back round again. Always on the lookout for tricks!

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Are the calipers refurbished or new?

Though I’ve not heard of this problem with the type 14 calipers, it definitely is a common problem with type 16s as used on GT6/Vitesse and is caused by the caliper seals returning the pistons too far.

If this proves to be the issue, removing the pads, carefully pumping the pistons out a bit, then levering the pistons back just far enough to squeeze the pads in will solve the problem, but it may well return as the pads wear down.

Nick

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Hi Folks, I'm sure the trailing shoes are correctly positioned, but I'll take the drums of and check.  when fitting new shoes etc, I have always moved the adjuster fully tight then stamped the pedal to ensure they both are hard up and then adjusted one click at a time until I can just get movement of the wheel, then again stamp on the pedal to centralise,  and finally adjust tight then back of one or more clicks until wheel runs clear. Yes, the nipples on the slave cylinders are at the top.   Not sure about the Fitchett's supplied caliper type but will check.   I think Tony's "clamp hose" idea may also indicate a master cylinder problem if the pedal is still unchanged, so watch this space.........Cheers Dave

 

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On 30/03/2020 at 14:38, poppyman said:

Dave, clamp all the flexi hoses off, if still the same it's mcr cyl

I did the clamp test, though had a firm pedal on the first one I clamped (which was the culprit), any reason for not trying one at a time, though leaving the previous ones clamped?.

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If it's the MC then fluid is getting past the seal; instead of being pumped it's escaping round the sides and so the pedal goes down with no effect at the brakes. Item 13 in this exploded diagram is the seal, just behind the piston that you can see if you look at the end of the cylinder (part 14). It's all held in with a circlip and if you feel up to the job you can replace it quite easily yourself. Kits are about £6 or so and contain all the bits you need to refurbish; just make sure you buy the correct version for your master cylinder. Where did you get the one you replaced the original with - was it new? Did it come with the proper length of push-rod? (part 15)

 

cylinderexpanded.jpg

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On 31/03/2020 at 20:05, johny said:

No don't think it'll be item 13 as fluid will come out and run down so more likely to be seal 8. This can sometimes be confirmed by the level going up in the reservoir when pushing on pedal...

Lol yes was picturing it back from the pedal in my mind but for some reason thought of it as a plunger ie same as the rear cylinders where the metal end pushes out, and not that it's pushed in... typical!

Incidentally one problem I've found this week, whilst trying to adjust the Estate brakes is that the axle lies on the chassis thereby causing additional friction to the brake shoes. It may be the new spring, a  proper Estate spring, that's causing the problem. I may have to drop it off the lift and onto axle stands under the hubs or shocks to raise them.

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