Mot Failure (wifey's Tin-top)

[enforcer]

Wifey's Rover 618 failed the MOT today because the brake pedal was too 'spongey'. Didn't come as a surprise to me, but I'm now going mad trying to get to the bottom of it! Here are the symptoms:

 

Brakes were always fine and rock solid until a few weeks ago. Then started to notice that the brakes would come on as normal, but the pedal would then give a little bit more, and travel a further inch or two before biting again. This has been getting gradually worse up to today.

 

There are no leaks anywhere in the system, hoses are fine and discs and pads have been replaced.

 

Each caliper has been bled and bled - no air in the system!

 

Today I swapped the front calipers for a pair sourced from the scrappy - no difference after bleeding. I also replaced one of the rear calipers with a reconditioned one.

 

This all leads me to one conclusion - Master Cylinder. I think the seals inside the cylinder had been starting to go, initially allowing sufficient pressure to activate the brakes, then allowing the pedal to give as fluid seeped past the seals. Over the last week or so (lots of frantic brake bleeding) I think the seals have deteriorated further and now the pedal goes to the floor.

 

The servo works fine. There's a master cylinder in the scrappies which I will fetch tomorrow and give it a go. Has anyone experienced this kind of problem? Does it make sense when I say that the seals in the master cylinder are giving up? Are overhaul kits for cylinders still generally available? Best quote for a new one is £125 +vat!!!!

 

Any observations / advice / tips gratefully received!!

[Guest chris brown]

This all leads me to one conclusion - Master Cylinder.

My guess is you have got it in one. It’s not an uncommon problem in fact we had a VW in yesterday with exactly the same problem.

[Guest robinj66]

Sounds like the problem I had with my Cortina master cylinder. Brakes would work fine most of the time but if you sat at traffic lightswith your foot on the pedal, it would gradually creep down until it hit the floor. A bit worrying . I changed the seals twice before concluding that the bore of the cylinder was scored (not badly) enough the cause the seals to be ineffective.

[Guest Petemate]

As said by others, spot on Rich. Had the same thing on a mini years ago. Over the years after wear of the seals/bore, all that is helping the brakes to work is the general build-up of crud. As soon as the fluid is disturbed, e.g. by bleeding or simply a fluid change, it all gets cleared out and the result is one is unable to get a good pedal. Hope the replacement cylinder is a good 'un. Problem is most kits these days cost nearly as much as a new cylinder, that is those you can obtain. I priced a kit for a tandem one on a Peugeot 205 for a neighbour a few years ago and it was over £90 - together with welding needed this promptly persuaded her to scrap the car.

Pete

[Peter Bell]

I managed to get a kit for the sierra cylinder from my local motor factors about 2-3 years ago, think it was about £20, the cylinder bore was ok. If you can find a kit I would check the cylinder bore is alright before you invest in it.

Peter

[Guest chris brown]

I haven’t refurbished a cylinder for years either master or slave. Its not worth the effort, the cash saved is little and the chances of it failing again in the near future is high. Bite the bullet Rich and go and buy a new one.

[andyparry]

You need to isolate the master cylinder from the rest of the system to verify the problem. Pop round your local motor factors and buy 3 or four brake nipples. Replace each of the pipes going into the master cylinder with a nipple. If the pedal still moves, then it is the seals.

[enforcer]

Folks, thanks for the help so far!

 

I have now got my hands on a replacement master cylinder from the scrappies which seems in fairly good nick. I've drained and removed the old one and cleaned everything up, and have now fitted the replacement after a thorough cleaning. Everything's all connected up but I can't go any further now because my little bleeder has gone swimming and won't be back for a couple of hours!

 

Hopefully I can update this as a success story a little later?

[enforcer]

This is not going well.

 

I have now replaced:

 

Master Cylinder

Front offside caliper

Front nearside caliper

Rear offside caliper

 

bled system again fully, and STILL the same, spongey brake pedal!!!! I have now parted with over a hundred quid (all parts were second hand though) and still nothing.

 

I have found that by clamping one of the front flexies I can get a firm pedal, but I'm not convinced the calipers are the problem. I've bought two new bleed screws and tomorrow I will start again, initially using these to block off the two pipe exits from the master cylinder to satisfy myself that the seals are good. Any more ideas folks? Keep 'em coming

[Guest Paul Kirby]

Here's an idea; whilst applying pedal pressure, check the flexi pipe(s) for bulging or ballooning. This will give a spongy pedal. I have known this to happen. If this is the case the flexi(s) will ofcourse need replacing.

Edited March 31, 2007 by axle man

[Guest Petemate]

Rich - you may just have highlighted it. Get an assistant, as suggested, to carefully check the flexi that you clamped. It only takes a raally small amount of "ballooning" to give extra travel.

HTH

Pete

[enforcer]

Now decided that the pedal is definately 'creeping' rather than just 'spongey'.

 

I've been perusing some other forums (Rovertech.net for one) and there is quite a bit of reference to the potential for damaging master cylinder seals by bleeding - the idea being that the master cylinder piston never travels the full length but when you bleed the brakes and push the piston to the limit of its travel this can damage the seals. Has anyone heard of this? I've always gone by the Haynes way, where you are specifically instructed to press the pedal down to the floor.

 

Does anyone know if the ABS unit could be to blame?

[Guest chris brown]

We never bleed the brakes by pumping the pedal to the floor for that reason but always use a pressure bleeder or the latest thing we have is a vacuum bleeder that sucks the fluid from the bleed nipples. BTW always when bleeding brakes start with the wheel furthest away from the master cylinder and work round getting closer

[Guest timswait]

Has anyone heard of this?

Yes I've heard of that, but I think it more applies to an old cylinder that is on it's way out anyway than a brand new one. I take it the spongeyness is happening on the drive? I had a friend with a loose wheel bearing, the disc vibrated and forced the piston into the caliper as the wheel went round, so everytime he needed to brake he had to pump the pedal.

I wouldn't have thought it was the ABS unit, more likely a bulging flexi, although if it's creep rather than spongey then that more suggests that you've got another duff master cylinder.

[enforcer]

I do hope not. Gonna sleep on it and tomorrow knock the master cylinder question on the head by trying Andy Parry's idea of blocking off the pipe exits with new bleed screws and isolating the master cylinder.

 

If doing this shows that the pedal is firm and no longer 'creeping' at least I won't be forking ou £125 + vat for a new one!