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MG Midget and Sprite Technical - GMC151 cap open or closed vent hole

Something I noticed before and then forgot about. The all plastic GMC151 brake master cylinder I have has a cap with a raised blind/closed/not-opened/not-drilled-out vent, same as the clutch master cylinder but without the vent hole.

Is this correct?

It's been like it for a number of years, at the time I thought it must vent in a different way, say through the seal.
Nigel Atkins

I can't see how it would vent through the seal.
Dave O'Neill 2

A small gap left in the seal or cap inner lip or both, just a thought at the time and then I forgot about it until now.

I think I only noticed the first time I wiped the brake and clutch caps and got a needle to make sure the vent holes were clear, and there it wasn't.
Nigel Atkins

Perhaps it doesn't need an air vent?
The partial vacuum created would help to pull the brakes off again when you release the pedal. Might help with the front discs?

Or maybe there is a pin hole out sideways through the ribbed edges of the cap?
GuyW

I believe Guy has hit the nail on the head. This came up in a discussion many years ago, although in the context of Fords I think.
The change from drums to discs resulting in the linings dragging because of the loss of the return springs neatly, if a little crudely, addressed by deleting the pinhole, just as Guy implies.
It used to be common to see MC caps with the breathers plugged with bits of matchsticks/self-tappers/whatever and I always understood this to be the reason for it.
Greybeard

Important then that the m/cyl is not overfilled. It would need a volume of gas (air) above the fluid to provide the compressible / expandable medium. (fluids being, for all practical purposes, incompressible.)

If it were 100% fluid then it would create an hydraulic lock - until something leaked!
GuyW

I'll have a look.

I was a bit worried seeing Guy's comment about overfilling until I thought it's been the same level for years allowing for it being on a lean.

I'll have to look through my paperwork to see how long it's been installed as I remember the car came with one mater as tin and the other plastic and they're both plastic now and I'm sure both changed at least once as with most things on the car.
Nigel Atkins

Could it be that it's the clutch master cylinder cap that doesn't have a vent hole?

Could I, or more likely someone else, have swapped the brake and clutch caps (I'm very careful about removing a cap as I don't want brake fluid drips in my paintwork or anything to get in reservoir).

Looks like I've had a (another?) false memory as I can find no record of changing the brake master cylinder, the clutch cylinders have been changed twice though, I've no need to look that up as it's a bitter painful burnt in memory.
Nigel Atkins

ETA:
I forgot to put both caps look the same except for lack of hole in vent chimney and that one has not another vent hole as I tested it by trying to blow through it (lips still sting and a nasty taste in my mouth - just joking I cleaned the cap thoroughly first).

Both caps on the underside have a clear plastic disc held in the centre but free at the circumference that can be spun (a diaphragm?), under this is an inner lip that has a v-nick in it 180 degrees opposite the vent (hole).
Nigel Atkins

To close this thread off I've opened up the hole that wasn't there.

I've put a hole in the cap that didn't have one.

I found nothing to suggest that it shouldn't have a hole in the cap so it looks like possibly another car part that slipped through quality control.

The tin can type of master cylinder has a hole in the side of the plastic cap, I know this cos it sez so in the Driver's Handbook and I remember checking those caps on my previous Spridget.
Nigel Atkins

This thread was discussed between 05/11/2017 and 09/11/2017

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