Clutch master cylinder ?

[PeterH]

 I have a very strange problem possibly with the clutch master cylinder. There is evidence of fluid on bulkhead under and around the master cylinder, and the reservoir level goes down, only the clutch. Fortunately I use silicone fluid so the paint is not affected. But how is it getting out ? It's a new cylinder and not leaking from the piston. Could it be expanding / boiling ? The loop of pipe is near the exhaust manifold but well clear of it.

Any ideas would be appreciated.

I should say that the level used to drop in the old cylinder but I put that down to a possible leak from the seals, but perhaps it wasn't and it was the same fault as I'm now experiencing.

Edited June 26 by PeterH
addition

[johny]

Cant see expanding/boiling but has the master cylinder got a leak. If piston seal is good could its casing have a crack or porosity?

[dannyb]

I would take the master cylinder off and bench bleed it. This will show up any leaks.  Just Google bench bleeding master cylinder for instructions.

Danny

[Pete Lewis]

unless as said there is a some crack (pretty unusual) there is only the piston tail seal or the outlet pipe

or out the cap

any where else and we're all lost on this 

ideas of boiling expansion would just overflow from the cap 

Pete

[dougbgt6]

Silicone does damage the paint, it takes the shine off and makes it go dull.

Two of us at East Berks have had the loop go brittle and crack due to the engine flexing. That's why the loop is there, they just didn't expect it to last 50 years.

Doug

Edited June 27 by dougbgt6
too to to

[PeterH]

31 minutes ago, Pete Lewis said:

unless as said there is a some crack (pretty unusual) there is only the piston tail seal or the outlet pipe

or out the cap

any where else and we're all lost on this 

ideas of boiling expansion would just overflow from the cap 

Pete

I don't think the casting is porous as there is no leak when the car is just standing in the garage and there is no leak past piston seals. But I will try pumping the pedal up and down several times while stationary. I think its coming out the cap, I'll also try tying a poly bag over it next time I go out for a run.

[johny]

The porosity could be in the pressure area so fluid only comes out during use? Also you could try running with a lower level in the reservoir to see if that stops it if from the cap. In fact is the cap sealing properly as its got to prevent the slop of fluid under driving conditions... 

[Peter Truman]

In the machining of the master cylinder ie the piston bore, I've seen an indented dimple on the back outside of the master cylinder where I assume the cylinder has been mounted in a lathe or similar to centralize to machine/create the bore, just wondering if that dimple is a little too deep? creating a fluid path, paper towel under the master would show this?

Is your delivery pipework & loop copper or the original bundy steel pipe, copper pipe can work harden and can split/fracture, hence here in Aus it is illegal to use copper in the auto brake system! 

[Colin Lindsay]

My brake cylinder leaks fluid (takes the paint off!!) and after a lot of exploration I realised it was coming out of the cap, due to being slightly overfilled - the level was fine until I replaced the front pads and pushing the pistons back raised the level too high. The cap is good, and the seal too, but it's still finding a way out so keeping it slightly lower than the level mark (and checking regularly) seems to fix it. This is one of the large plastic-reservoir cylinders on the late 13/60s, but another problem too is that replacement cylinders aren't properly angled so as to sit upright when fitted, and as they slope to the front then fluid runs out if too high. 

I also wonder if the level is even slightly high then sharp braking may make the fluid surge to the front and therefore spill slightly? Just a thought.

[johny]

Yes the cap has to have a hole in it somewhere for venting so fluid can definitely come out of it even with a good seal...

[Mjit]

Clean everything up to remove any existing grease/oil/fluid, then start wrapping up the master cylinder with masking tape like a Hollywood mummy.  Leave overnight/check for any staining/discolouration of the masking tape.  If all good go for a short run and check again.

Hopefully you'll be able to trace where the fluid's coming from...

[Puglet1]

Hi Pete, I’m not trying to be negative but I’m not sure about using a polythene bag. The reason I say this is An old trick to prevent the braking system from draining out when the pipes were disconnected was to place polythene under the master cylinder cap - which created a vacuum due to the breather hole being blocked off.

So if the clutch pedal operation feels unusual, the polythene bag might be the cause.

 

[PeterH]

I suppose I ought to own up to the cause. 🤪

The pipe union not fully tight. Tight enough to bleed but a very slight leak under pressure. Perhaps I should re-post under 'Blond Moments'.

(reminds me of the old definition of an expert: X an unknown quantity, Spurt - a drip under pressure.)

[Colin Lindsay]

16 hours ago, PeterH said:

I suppose I ought to own up to the cause. 🤪

The pipe union not fully tight. Tight enough to bleed but a very slight leak under pressure. Perhaps I should re-post under 'Blond Moments'.

(reminds me of the old definition of an expert: X an unknown quantity, Spurt - a drip under pressure.)

It's the old balance of 'tightening enough to seal but not enough to strip the threads on the alloy master cylinder' scenario. Been there!