Hi all,

Since I've had my front disc brake conversion I've had an issue with the brakes sticking on. The mechanic had a look and confirmed that the assembly seems fine and there's no obvious reason why they should be sticking.

When it happens the pedal is "rock" solid and the brake lights are on, which makes me think it's an issue with the master cylinder.

I've heard a few people suggest that a conversion to the bigger 126 brake master cylinder is a wise mod when you go to disc brakes, others say that it's unnecessary, so my first question is related to that.

If I assume it's faulty and I'm going to fit a new master cylinder anyway, should I go for the larger 126 version?

Second question, if the answer to the above is yes, is it a straight swap or do I have to modify the car in any way for the 126 cylinder to fit. I can see that it's physically quite a lot bigger.

Thanks in advance for any words of wisdom!

I believe that you need to cut away some into the spare wheel well.
Ricambio UK do a very expensive upgrade that does not require fabrication.

I would consider that the original master cylinder is faulty....I'd be inclined to replace "as is" and see how you get on.

I have a brand new "in the box" master cylinder if you'd like it

The 126 cylinder is dual circuit and so longer - requires a hole cutting in the panel where the spare wheel well sits I believe.


It sounds like you may have an issue with piston stroke. To 'recharge' the fluid, the piston and seals pass over a replenish hole from the brake fluid reservoir. with your foot off the pedal, this hole is exposed to the main cylinder. As soon as the piston moves a very small amount (ie you press the pedal) this hole is blanked off allowing you to transmit pressure through the brake system. When you release the pedal, the internal springs allow the port to be uncovered, effectively allowing the brake pressure to release to atmospheric pressure and letting off the brakes fully. Wikipedia has a picture which is probably more clear than the above https://en.wikipedia.org/wiki/Master_cylinder#/media/File:Master_cylinder_diagram.svg
It sounds, if your pedal is rock solid, that the spring or the port may not be working correctly. There is the issue discussed with Tom about the pedal rod length being critical - if the spring cannot return the pedal and piston fully you wont uncover the port to release the pressure. If the rods too short, you have a spongy/ long pedal as you don't cover the ports as quickly.


Sounds like you have a dodgy cylinder/ spring. check its fully returning with no obstructions (remove the rod from the pedal) and that the feed port from the reservoir is not blocked - I once had a clutch slave that was leaking so I replaced but I couldn't bleed by pumping the pedal as nothing came out of the bleed screw after a little dribble. I used a pressure bleeder and the pedal as well and eventually loads of gunk (old bits of rubber seal I presume) came flooding out with the fluid - must have been in the clutch lines and had blocked up the new slave cylinder bleed.
hope that helps.

cambio said:

Sounds like you have a dodgy cylinder/ spring. check its fully returning with no obstructions (remove the rod from the pedal) and that the feed port from the reservoir is not blocked

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Thanks for the advice. It happened once before, just after the conversion, but the brakes were engaged just a little and I was able to creep the last 2 miles home. The mechanic checked the discs and calliper, pipes and connections and all seemed to be ship shape.

It's been driven a few times since and has been fine, but today the brakes locked on.... hard, and I had to be recovered. It's been towed to the garage that fitted the new disc brakes.

I've found the uprated master cylinder kit. Erm..... yes.... expensive!! So in the first instance I'll just get them to switch the master cylinder like for like and get them to check the spring/rod/pedal assembly. Ricambio are on holidays until the 5th June, so I've ordered one from Axel Gerstl. They tend to be quick with delivery.

If that doesn't fix it I'll go for the Rolls Royce master cylinder modification kit!

AndrewHarvey said:

I believe that you need to cut away some into the spare wheel well.
Ricambio UK do a very expensive upgrade that does not require fabrication.

I would consider that the original master cylinder is faulty....I'd be inclined to replace "as is" and see how you get on.

I have a brand new "in the box" master cylinder if you'd like it

Click to expand...

Thanks Andrew, sadly I read your email just after I ordered the master cylinder from AG, but thanks for your kind offer.

Yes, I think I'll switch for a like-for-like cylinder in the first instance. Check the push rod and springs, and if that doesn't work I'll consider the Ricambio upgrade.

I fitted the expensive Italian supplied master cylinder and documented my struggles on this forum.
The brake pedal rod was too short - giving me an unacceptable long pedal travel.
I ended up making a clevis bracket with a 10mm thread and a 10mm studding turned to a blunt end?.
It allowed me to adjust the pedal rod to give me a perfect pedal travel
My fancy Italian master cylinder gives 30% more stopping....and i have discs all round....

Your brake stuck on problem is because the master cylinder piston is not able to return to its proper rest point.
I know nothing of the 500 set up so if the following suggestion is useless please forgive.
If push rod is adjustable shorten it.
Or put washers between master cylinder bolt holes and brackets , making it slightly further away from pedal.

On the forum there is a x1/9 post about the very same thing , except it was pushrod from servo not adjusted correctly, I expect no servo for you which makes things easier.

Peter, I have a dual circuit master cylinder from the 126 and disc's on the front with the later 126 drums on the rear. If you wish I can photo the fitting for you. It will be tomorrow at the latest - let me know.

Ian.

Bleeding Knuckles said:

If you wish I can photo the fitting for you. It will be tomorrow at the latest - let me know.



Ian.

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Thanks Ian, that would be very useful.

Hello Peter, Here are the promised photo's sorry for the delay - other stuff got in the way The first photo shows my reservoir, second the rather crude fitting underneath, I think there has been just a bit of modification with a drill The next photos show the master cylinder and the proximity of the forward bulkhead and the (again) crude mod to the main bulkhead to get it to fit. Not a lot of room as you can see. I have Fiat discs on the front that do not involve adaptors (not sure where they came from) the brakes do need a firm shove to work and I am uprating the piping with a steel braided kit later on to improve the feel. If you have any questions please ask.

Ian.

So here's the update.

The mechanic that I used to install the new master cylinder (straight swap for a standard unit) also conducted a thorough check of the mechanism from the pedal to the cylinder, to check for various issues that might prevent the pedal/pushrod/spring from operating correctly. And this is where it gets strange!

He found a piece of rubber tubing that had been cut to about 20mm in length 'wedged' tightly into the pedal assembly where it pivots (actually in the footwell). There's no way that it could have got there by mistake, it must have been pushed in there at some stage in the car's history as a crude way to adjust the amount of travel in the brake pedal.

It must have been 'calibrated' to work fine with the old drum brake system, but obviously the tolerances are closer with the disc set-up and it's very likely that the lack of travel in the pedal (before the new disc brakes engaged) has at least contributed to my intermittent problems since the conversion was made, if not been the cause.

Driving the car home there's now a sensible amount of travel in the pedal before the brakes engage. In hindsight, just since the conversion there was virtually none, so as soon as you touched the pedal the brakes engaged. This now feels a lot more natural.

Anyone seen this before? Here's an image of what was wedged in there!

Glad it was such a simple solution to enable the master cylinder pistons to return to their correct "at rest point"

As I have mentioned before--one has to be VERY careful with these Italian 'fully restored' descriptions. Some of the crap appearing just has to be seen to be believed! I have been advised (by somebody very knowledgeable in the 500 business) that the flood of 'fully restored Italian 500s' has really depressed the market, and that most of what is coming in (cars AND parts) is total rubbish and has 'front-door warranty' (i.e. as it goes out of the door, warranty ends). This bodge almost matches Ian's (Bleeding knuckles) fuel gauge fitted to act as an oil-pressure gauge!

the hobbler said:

As I have mentioned before--one has to be VERY careful with these Italian 'fully restored' descriptions. Some of the crap appearing just has to be seen to be believed!

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Indeed! Scary stuff. Even scarier is that this car has been in the U.K. since the late 90s according to its history and was "fully restored" by Channel 5!!

I had forgotten that 'little' detail Peter. I bet Bernie worked to a higher standard in his previous employ!! The fact that that 'little detail' was overlooked makes you wonder what else was 'overlooked' Having met Peter and looked over his car and discussed his problems with him makes one wonder as to how good some of these 'restoration' programs really are!

the hobbler said:

I had forgotten that 'little' detail Peter. I bet Bernie worked to a higher standard in his previous employ

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Ha ha! Indeed Tom, concrete boots spring to mind!

Maybe this piece of hose has been there for many, many years, and pre-dated their work.

Thankfully the engine and suspension (and much more besides) has been 100% stripped, modified and rebuilt since then. There's very few corners, nooks and crannies of this car left that hasn't had a spotlight on it since the guys at Classic Car Rescue had their hands on it. Who'd have thought that this little trick was hiding in the footwell!!?? Amazing!!