MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Weeping bleed valve

As I was bleeding the brake system anyway, I took the opportunity to renew a couple of rusty looking bleed nipples.
I now find I have just a tiny weep of brake fluid from the threads of the nipples. Thinking it through, I imagine that the seal does not come from the threads, but from the tapered head sitting in an aperture in the slave cylinder body.
If that is correct, could the weep be just excess fluid sitting in the location between the head and the threads, and is nothing to be concerned about? Or is that wishful thinking?

I've never noticed it before but I would normally just tighten the bleed valve and move on, i.e. Not keep going back to check.

Thanks.
Graham V

It might be worth getting an assistant to depress the brake pedal while you look for signs of leaking.
Dave O'Neill 2

Give the nipple a spray with brake cleaner press the brake pedal and see if there is a weep
Bob Beaumont

Graham,
you're right it's the taper that gives the seal and you're probably on wishful thinking. I had problems with leaks from the old nipples and their replacements so I'd suggest the following.

As Dave and Bob suggest.

Have the correct size spanner(s), new nipple spanner sizes can be imperial or metric, check you have fully tighten down the nipples (without overdoing it obviously).

Recheck the tightness of all the nipples a couple of times after a couple of reasonable length journeys.

If you still have a weep report back.
Nigel Atkins

If a nipple leaks, remove it and check the pointy end for debris or scouring, and also check inside the casting. Often a small speck of dirt on the seating hole will prevent the nipple from sealing.
GuyW

Thanks for the feedback. I have tried the suggestions without success. Having said that, one side seems to be behaving itself now, but not the other. I have replaced the new one with the original nipple in case it made a difference, but doesn't seem to.
(Interestingly, on googling the problem, I was surprised about the amount of discussions over the rights and wrongs of using ptfe tape, when as Nigel has confirmed, the seal is not on the threads but on the tapered head).

Anyway I still have a tiny weep on just one side. It is not a drip, no puddles, etc, and it is only detectable by pressing my finger on the point between the thread and the slave body, and detecting a small line of fluid on my finger.
As I mentioned earlier, I dont think I would normally have checked so carefully and would have been happy in my ignorance!

I am still wondering if it could be excess fluid sitting beyond the tapered head. I guess if I wait long enough I will know!


Graham V

Dry it off with tissue and leave it as it is for a couple of days before checking again. If possible, drive it a bit and then re-check. It will likely cure itself, but if it doesn't it will become more apparent where the fault actually arises.

The purpose of using PTFE tape isn't usually to stop fluid leakage as you have correctly realised, but to prevent air working its way up past the threads as you bleed fluid out of the nipple orifice.
GuyW

The nipples to the cylinders on the rear brakes on my '73 midget were 7mm spanner size, I had to go and buy a spanner that size. I was quite surprised with two of the four new nipples how much nipping up I could do with various types of spanners in different positions on the nipple to obstructions.

I took the advice and put grease around the top of the nipple thread to prevent air getting back down the thread.

Do check carefully where the leak is (if it doesn't disappear with various efforts to tighten the nipple) as I had to replace the supply pipe too after it soiled my new replacement shoes.

And do check that your reservoir cap vent hole is clear - and that the cap actually has a vent hole!
Nigel Atkins

Wealth the threads in plumbers Ptptfe tape
1 Paper

Reporting back.

First, I really dont think ptfe tape is the answer. I know lots of people do suggest it, but the seal isn't at the threads, its at the conical head. If it doesn't seal properly at the conical head, then the fluid has two routes out. One is via the thread but the other is through the hole beyond the head, and down the internal volume (see pic).

The good news for me, is that it appears my bleed valve's seems to have stopped weeping. At least its been dry for the last 24 hours.

My reckoning is that after bleeding, fluid sits in the area between the threads and the conical head and it very slowly works its way out. Once its gone, its gone.

At least thats what I am hoping!

Graham
Graham V

Forgot to attach picture - pretty sure we all know what it looks like, but it does make the point clear as to why ptfe tape will not work, if the conical head seal fails.


Graham V

Quite right Graham. PTFE tape on the thread does nothing for the seal.

But....

What it can do is prevent the threads from galling, although PTFE paste (aka: SWAK) is better than tape for that. Especially on stainless threads which can be prone to galling.

Your explanation sounds very reasonable, plus there's the volume of fluid in the bore of the nipple....
Greybeard

Even the slightest weep will result in a pedal that feels spongy and will need a pump to firm it up if left for more than a few minutes
When you returm to the car after its been stood make a mental note of how the pedals feels on the initial press.
If there is a leak of any sort it will be felt
Regards
Stuart
S G Macfarlane

<<Even the slightest weep will result in a pedal that feels spongy>>

Sensible comment Stuart, although that isn't strictly true. If the system slowly leaks fluid out, but doesn't draw in air then for a good while the pedal will remain firm, not spongy. This can of course be misleading as eventually if this isn't detected and the reservoir isn't being regularly checked, the reservoir will run low it will draw air in at the master cylinder and this will result in a sudden and very likely catastrophic "pedal to the floor" situation.

It's why regular check of the m/c fluid level is essential on cars like these which don't have a fluid level warning light system, unlike a modern car.

AND,

PTFE tape (or grease) on the thread is used when bleeding the old fashoned way with a short length of rubber pipe with its end immersed in a jar of fluid. When pumping the brakes this way, when one takes one's foot off the brake pedal it should just draw air-free fluid back up the rubber pipe, but if the nipple threads are leaking air it will gulp air back in, making proper 100% bleeding very difficult. Its nothing to do with sealing the bleed nipple once it is tightened shut.
GuyW

Guy
Im not sure if your two observations are consistent - and im not sure if i can explain myself either.
If air can get sucked back through the threads when you release the pedal, surely air can also get sucked back through a leaking bleed nipple when the brakes are released?
In theory the suction back through the leaking nipple would be even greater than through leaking threads because when the nipple is open and bleeding is in progress some of the suction is counteracted by movement of fluid in the bleed tube as stated above?
S G Macfarlane

Maybe, but l said in a slowly leaking system, not necessarily a leaky bleed valve. If the system is leaking slowly overnight or when the car is parked up, then there isn't any "suction" as such to draw air in and the level in the reservoir will just gradually drop.
My point was to check reservoir levels regularly and not rely on the pedal to go soft to alert you to a potential leak.
GuyW

This thread was discussed between 08/04/2018 and 18/04/2018

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.