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Why is there play in the clutch master cylinder push rod?


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Hi All,

I can move my TRW clutch master cylinder push rod in by 2mm before it contacts the piston. As this translates to maybe 10 or 12mm free play at the pedal I thought it must be a fault.

The clutch pedal spring holds the m/c push rod away from the piston so having a different length or adjustable push rod will make no difference at all. I would have expected to push in the piston slightly to insert the circlip but this is not the case. There is a gap between the washer and the circlip. (see photos)

I have received a new Girling master cylinder and push rod this morning and it exhibits exactly the same characteristics.

Why does the design include free play? It baffles me.

In my mind it would be better to have a slightly longer spring/piston assembly so it always touched the push rod and eliminated any free play.

Grateful for your thoughts

Regards

Les

Mc spring.JPG

Inset washer in mc.JPG

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Les

Interesting yours has two seals, my Girling 0.7" has only one. Maybe TRW use two?

Or have you the right cylinder?

Maybe someone can confirm.

Andy

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Two seals are good. Some models actually have one, which causes the steel piston to directly rub against the interior wall. With two seals, one on the piston and one behind the piston, actually helps in guiding the steel piston inside this aluminum housing. The main issue for failure in these MCs are not the rubbers affected by DOT3 or 4 but scratches which in turn damages the rubber and results in fluid passing by the push rod. Also, fluid turning grey is the first indicator for steel rubbing on aluminum.

Jochem

IMG_5398.JPG

Edited by JochemsTR
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Hello Les

Your MC photo and that of Jochems show a slight difference. Both should fit up flush to the mounting bracket but yours looks like there is a slight 'shoulder' where it would fit up to the hole in the mounting bracket. If the cylinder was 2mm further forward then wouldn't the play be taken up? However I have just checked both my brake and clutch MC's and both have play so I reckon this is normal. I have a 3/4 inch clutch MC on my 4A. Would a 3/4 inch (0.75 inch) MC be a better bet for you rather than the 0.70 inch one you have?

Keith

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I’ve been trying to understand how the M/C works. With the main reservoir at the end and the take off to the slave before it. I’m guessing the initial movement of the internal cylinder must close off the main reservoir, and then subsequent compression of the internal spring/cylinder/seal(s) expels the hydraulic fluid out to the slave cyl. The valve at the end of the internal piston must close inside the main reservoir with the first few millimetre of movement. Is this the reason that it needs some free play so that it can return to open? I’m also thinking if I could understand how it works it might make bleeding easier? Bleeding the clutch is always a challenge for me. 

How the M/C works maybe obvious to every one on this forum, but I’ve been scratching head to understand how it works. 

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Robert, thanks for the link. It is a good description and in principle it makes sense. I still think the TR M/C differs in that the reservoir is at the end, so how do the seal(s) close off the reservoir before they pressure the slave. My guess is the valve at the end of the internal piston is closing the reservoir when the pedal first moves, and then the main seal pressures the fluid to the slave. When the pedal is released the internal piston must come back with the spring and I’m guessing it needs some free play so that the valve can also open to the reservoir. Just my guess. Does anyone have a cut through cross sectional drawing of the clutch M/C - just wondering if the internals are more complicated??

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12 hours ago, Martin50 said:

Does anyone have a cut through cross sectional drawing of the clutch M/C - just wondering if the internals are more complicated??

Single acting brake master and clutch master are nominally the same internally as they both only have to do one thing which is pump fluid, the bit of free play is to ensure the cylinder fully returns otherwise there will be pressure held in the line, Ive seen this cause problems on earlier cars that have a pedal stop bolt on the mounting that has been screwed in too far in an attempt to lower the pedals.

Stuart.

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Stuart, thanks. I had been looking for info on clutch m/c but searching on internet and including brake m/c there is a good clip on you tube with cross section - again only generic but it does given a guide to how it works. Can we share you tube links on here?

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50 minutes ago, Martin50 said:

Stuart, thanks. I had been looking for info on clutch m/c but searching on internet and including brake m/c there is a good clip on you tube with cross section - again only generic but it does given a guide to how it works. Can we share you tube links on here?

Yes when you start to write a post you`ll see on the top next to the link paperclip there is a dedicated Youtube link

Stuart.

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Thanks everyone.

Good to know there should be bit of free play. Thanks Stuart, it makes perfect sense not having any pressure held in the line. I have now fitted my new Girling master cylinder which only had a single seal. In retrospect, maybe I should have asked the question about free play on the TRW before assuming it was faulty and buying the Girling. I do like the idea of the double seal on the TRW. I have now replaced the master cylinder with new Girling, the slave cylinder with new Lockheed (mounted the correct way up), new flexible pipe and new 82mm master cylinder push rod but I still only get around 12mm throw at the slave cylinder. I guess I need to look again at the bleeding.

I have left the clutch pedal jammed against the floor overnight and will check later if it has made any difference.

Does anyone have any foolproof tips on bleeding the clutch system? I started with a 'Vizibleed' and then moved onto two person.

Thanks

Les

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Les,

I’ve recently bled the clutch system. I’m not a great fan of “ezy” systems and employed my trusty assistant (aka my wife). My approach is not to stop the process the first time I see “no more bubbles”, but to continue for at least three more pedal depressions. It’s surprising how often more air is expelled. 
Good luck!

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Hi Paul,
Thanks, I think that is good advice. I will try again with my brother in law over the weekend as I am not too sure about how good my wife is at holding her foot flat to the floor.

My master cylinder push rod is moving 30mm which, allowing for the 2mm free play, gives 28mm of effective movement which should translate into 14mm at the slave cylinder push rod.

I can't think of anything else to do to be honest.

Regards

Les

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On 12/2/2021 at 11:06 AM, Les Pettitt said:

Hi Paul,
Thanks, I think that is good advice. I will try again with my brother in law over the weekend as I am not too sure about how good my wife is at holding her foot flat to the floor.

My master cylinder push rod is moving 30mm which, allowing for the 2mm free play, gives 28mm of effective movement which should translate into 14mm at the slave cylinder push rod.

I can't think of anything else to do to be honest.

Regards

Les

Make sure there is no lots motion in the pedal clevis and the slave clevis too. When bleeding make sure you have quality bleed screws as the cheap ones are such a poor fit that when you slacken them to bleed your drawing air back in around the thread. I always use Brass Automec ones .https://automec.co.uk/collections/fittings/products/brass-bleed-screws-unf?variant=13048922079255

Stuart.

Edited by stuart
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Hi Les.

I know you know how to work your C/M/C pushrod travel mathematically, but snip attached.

I am still very curious where you measured this travel, hope you soon get your problem sorted.

My brakes are now completely overhauled, I only have the refilling of the brake fluid left, fingers crossed I don’t have any leaks or problems.

Mick.

252487793_Lessnip.PNG.ba374844daee0f49a9f8703dd2431cf9.PNG

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My full proof single person bleeding method involves attaching a 3- 4m long piece of silicone tubing to the bleed nipple with the other end poked into the master cylinder in such a way that I can see the tube from the drivers seat. I undo the bleed nipple and sit in the car with the radio on and gently pump the pedal watching the fluid going past in the tube until I see no bubbles. Then I do up the bleed nipple. Because it is a closed circuit you don't need to keep getting out to top up the master cylinder.. 

cheers

Tim

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8 minutes ago, Tim D. said:

My full proof single person bleeding method involves attaching a 3- 4m long piece of silicone tubing to the bleed nipple with the other end poked into the master cylinder in such a way that I can see the tube from the drivers seat. I undo the bleed nipple and sit in the car with the radio on and gently pump the pedal watching the fluid going past in the tube until I see no bubbles. Then I do up the bleed nipple. Because it is a closed circuit you don't need to keep getting out to top up the master cylinder.. 

cheers

Tim

Yep that will do it I have said before you need to restrict the fluid plain pipe will not get all the air out as it will find the easy way out 

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Hi Stuart,

The pedal was drilled out to 3/8 during restoration and I have drilled the master cylinder push rod to match and new 3/8 clevis. There is no play at all there. New slave cylinder push rod and clevis with no detectable play. The bleed nipple came with new slave cylinder but I do appear to get leakage of fluid from the nipple thread even when the nipple is only turned 90 degrees so a point for me to look at further. I have the bleed nipple from my original 1973 Lockheed slave cylinder so I could try that in my newly refurbished Lockheed slave. I do not know the origin of the bleed nipple that came with the refurbished slave but if you recall my post on incompatible TRW nipples with Girling brake cylinders I can fully believe I may have  a problem there.

Tim, I love your suggestion of a closed circuit. It is such a pain when I am under the car to keep getting up, walking round, topping up and going back under the car. It is surprising how quickly you can empty a master cylinder if not paying attention. Brilliant thanks, I'll try that.

Thanks everyone, I have renewed enthusiasm.

Regards

Les

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31 minutes ago, Les Pettitt said:

Hi Stuart,

The pedal was drilled out to 3/8 during restoration and I have drilled the master cylinder push rod to match and new 3/8 clevis. There is no play at all there. New slave cylinder push rod and clevis with no detectable play. The bleed nipple came with new slave cylinder but I do appear to get leakage of fluid from the nipple thread even when the nipple is only turned 90 degrees so a point for me to look at further. I have the bleed nipple from my original 1973 Lockheed slave cylinder so I could try that in my newly refurbished Lockheed slave. I do not know the origin of the bleed nipple that came with the refurbished slave but if you recall my post on incompatible TRW nipples with Girling brake cylinders I can fully believe I may have  a problem there.

Tim, I love your suggestion of a closed circuit. It is such a pain when I am under the car to keep getting up, walking round, topping up and going back under the car. It is surprising how quickly you can empty a master cylinder if not paying attention. Brilliant thanks, I'll try that.

Thanks everyone, I have renewed enthusiasm.

Regards

Les

One final trick with this method is that when you finish bleeding put two food bag type clamps on either end of the tube to keep it full of brake fluid. Then it is all ready to go next time you use it and it stops it dripping and damaging the paintwork (if you still use DOT3 or 4).

Tim

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Hi Tim,

Thanks. I've used silicon for the past 20 years or so. I have several plastic master cylinder caps so I think I will drill one to take the plastic pipe so it doesn't fall out of the master cylinder.

Or do just use a peg?

Mick, I put a tape measure against the top of the clutch pedal while my wife held it to the floor. She then released the pedal gently and the tape was pushed in 30mm.

For the slave cylinder I hooked a tape to the end of push rod and got her to press the clutch pedal while I held the tape still.

Regards

Les

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Les,

Modified top would work well, have also used a cloths peg :-)

Tim

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17 minutes ago, ntc said:

Les

If you are getting fluid around the threads you will never get the air out as you draw it back in when pumping even more so with silicone 

Precisely my point about poor fitting bleed screws.

Stuart.

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