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Bleeding problem


Reg
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I'm on the last stages of a complete rebuild of a Herald 13/60 and cannot bleed the brakes. I've replaced all the cylinders, disc calipers and copper lines. Two years ago I also replaced the master cylinder for a new 5/8 type as sold by Rimmers etc. equivalent to GMC220. After trying all bleeding methods in desperation I got a mobile mechanic to have a go. He was also unable to bleed the system. He the clamped off the front hoses and declared the master cylinder faulty or wrong type. ( The brakes appeared ok with the hoses clamped off ). has anyone else had problems with these master cylinders ? I notice they are made in Taiwan and marked 5/8.

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Assuming the front flexihoses are new (as they can also cause this effect), I think you'll find the problem is not bleeding but the calipers themselves.

I've now come across this problem 4 separate times, 3 times on type 16 calipers (Vitesse/GT6) and once on Type 14 where the calipers have been supplied brand new.  Problem seems to be either that the seal compliance is wrong or the piston finish too rough.  This causes the pistons to return too much after the brakes have been applied and prevents you ever getting a decent feeling brake pedal.  We wasted a fair bit of brake fluid discovering this on our Spitfire.

  You can check this by seeing how slack the pads are in the calipers after the brakes have been pumped up to the point where you have a pedal. They should be snug up against the disk but with only very light drag on the disc.  I suspect you'll find that they are still very slack. A solution, though sometimes only temporary, is to remove the pads one at a time and carefully pump the piston out (using the brake pedal) until there is no longer room for the pad and anti-rattle shim and then carefully lever the piston back until you can just get them in.  Repeat for other three pistons.

I reckon this'll give you a brake pedal that feels something like right.  However, the problem may persist and re-occur as the pads wear down and the calipers fail to properly self-adjust, giving a progressively longer pedal.  Easy enough to sort as above, but they shouldn't be like this.

Nick

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1 hour ago, Pete Lewis said:

are you sure the Mcyl is returning and there is free play on the push rod

if it is held open the reservoir will not refill after a stroke 

Thanks I will check that

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You said above that

2 hours ago, Reg said:

 ( The brakes appeared ok with the hoses clamped off ).

and your problem is that

14 minutes ago, Reg said:

The brakes only bleed so far---the pedal stillremains spongy

Does that mean that the pedal felt firm, no sponginess, when the hoses were clamped? If so then your mechanic is completely wrong to blame the master cylinder and I would lay odds on Nick being right.

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If the brake were OK with the hoses clamped off, and the air had been bled out of the rear brakes, then I agree with Nick and Rob. Like Nick I have had a similar problem in the past with spongy piston seals in the calipers. It felt like there was air still in the system. A spongy brake pedal. Nice!

Dave

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4 hours ago, NonMember said:

You said above that

and your problem is that

Does that mean that the pedal felt firm, no sponginess, when the hoses were clamped? If so then your mechanic is completely wrong to blame the master cylinder and I would lay odds on Nick being right.

Yes that is right---I will try Nick's trick with the calipers

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2 hours ago, AidanT said:

One other thing, as it's all new, have you plumbed all the slaves in with the bleed nipple at the top and not at the bottom?

Aidan 

Yes I learnt my lesson on that some years ago when I built a Teal kit car ( wish I never sold it as now worth around £40 k) I fitted the front calipers upsidedown  and it took ages to sus out what i'd done !

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7 hours ago, Nick Jones said:

Assuming the front flexihoses are new (as they can also cause this effect), I think you'll find the problem is not bleeding but the calipers themselves.

I've now come across this problem 4 separate times, 3 times on type 16 calipers (Vitesse/GT6) and once on Type 14 where the calipers have been supplied brand new.  Problem seems to be either that the seal compliance is wrong or the piston finish too rough.  This causes the pistons to return too much after the brakes have been applied and prevents you ever getting a decent feeling brake pedal.  We wasted a fair bit of brake fluid discovering this on our Spitfire.

  You can check this by seeing how slack the pads are in the calipers after the brakes have been pumped up to the point where you have a pedal. They should be snug up against the disk but with only very light drag on the disc.  I suspect you'll find that they are still very slack. A solution, though sometimes only temporary, is to remove the pads one at a time and carefully pump the piston out (using the brake pedal) until there is no longer room for the pad and anti-rattle shim and then carefully lever the piston back until you can just get them in.  Repeat for other three pistons.

I reckon this'll give you a brake pedal that feels something like right.  However, the problem may persist and re-occur as the pads wear down and the calipers fail to properly self-adjust, giving a progressively longer pedal.  Easy enough to sort as above, but they shouldn't be like this.

Nick

Thanks Nick for your help. I found one piston stuck in on the caliper nearest the master cylinder.

I sorted that and the brake pedal is now a lot firmer. I'll look at the other side tomorrow. At least I can drive the car for it's first MOT  now  following rebuild.

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I had exactly the problem Nick has described, although without spongy brakes - just a long pedal. Rectified a few months ago in the way Nick has suggested and not noticed the pedal growing longer as the pads wear, but I'm very light on brake pads anyway, often doing 50k on a set on the modern, which is 20 years for the GT6!

Gully

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Glad that worked. As regards whether it's a permanent fix or not seems to vary from calliper to calliper. On our Spit, actually with 16PB callipers, initially the pedal was virtually on the floor. After repositioning the pistons the pedal was nice and high, but their natural self setting position gives a pedal about 1/3 way down, which is too long imo. They can be reset as before, and have been about 3 times now (approx 5k miles) but always gradually work back to this position. Wheel bearings are fine and not the cause.  Wondering whether to try new seals or residual pressure valve to sort the problem once and for all.

A friend has a very similar issue with his Vitesse, but worse as his natural self setting position is near the bottom of the pedal, yet another man I know has had no further problems since the initial reset. Maybe just because he's hardly done any miles.....

Needs feeding back to suppliers as is a defect on a safety critical item and seems to be quite widespread.

Nick

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