Brake master cylinder

[Classiccars]

Hi had a brake master cylinder der from the club.Originally it was filles from a resovoir in the engine.I was advised to fill from the cylinder itself.So as the resovoir is redundant I was wondering if it may be sucking air in as the brakes are still poor.Wpuld it be worth finding a blanking screw to block it off at the cylinder itself and remove it as no use now.

[philthehill]

The remote reservoir should not allow air to be sucked into the cylinder as the level of fluid should be still be above the brake cylinder piston fluid entry.
It will do no harm to remove the remote reservoir and fittings. Just replace the modified rear blanking plug with a standard blanking plug and copper washer. If care is taken just replacing the rear plug should not require the brakes to be bled but you will loose some brake fluid out of the cylinder.
If you remove the remote reservoir make sure that you fit a standard (with vent) master cylinder filler cap.

[simmitc]

In what way are the brakes "still poor"?

Not sure why you would want to fill from the cylinder or remove the remote reservoir, which most people pay money ot buy and fit as it's much easier than grovelling around in the footwell; and also less likely to admit dirt to the system. However, it is your choice and I will not criticise, just curious. As usual, Phil's advice on how to do the job is pretty well perfect; I just question why?

Are your brakes standard drums? If you have discs, then you should keep the additional fluid capacity of the remote reservoir. Do you know whether the "top hat" seal is fitted to your new cylinder? It should be standard fitted for drums and modified for discs.

[Classiccars]

Thank you both for the advice.Very useful.No idea what a top hat part is .All I know is it is the cylinder from the club.The mechanic advised filling the resevoir the oil would leak from the vent on top of the cylinder at least he thought that was happening on the old cylinder.All I know is it was going somewhere as I was constantly filling the resovoir being the reason I changed for a new one.Have not filled it with the new cylinder on.Rear has discs and front is drums.All pads are new so could still be air in the system as I would hope the brakes were adjusted when they were changed.But I guess worth checking when we re bleed the system.

[simmitc]

When using the remote reservoir, the cylinder should be fitted with an unvented cap to prevent fluid loss as you describe. The part is readily available from most suppliers. In the early days, Araldite and a self-tapping screw were used to sealthe vent in the standard cap.

It is also essential that the gasket on the cap seals properly.

To maintain correct balance, most systems will use discs on the front and drums on the rear, or discs all round.

Is your rear axle a standard Minor one, or perhaps it's been swapped? What other modifications have been made to the car? Have you seen fluid leaking from the cylinder cap, or could it be a leak elsewhere?

I am concerned that this is not a standard system, and brakes are a safety critical item. I think that we need some more information about the history of the car and modifications. Why did yo order a new cylinder - what elase was wrong with the original?

Search for "top hat" in this forum and you will learn about the seal (look at the posts about brakes, not valve guides).

[philthehill]

The 'Top Hat' seal is inside the master cylinder and keeps a low residual brake line pressure (8 lb/in sq) so as to keep the wheel cylinder seals against the cylinder walls. When disk brakes are fitted the 'Top Hat' seal has to be removed as any line pressure will keep the pads against the disc leading to overheating of the pads/discs and ultimate brake failure. Removing the line pressure appears to have no detrimental effect on the wheel cylinder seals.
When a remote reservoir is fitted the master cylinder vented filler plug has to be replaced with a non vented plug. If you do not replace the vented plug brake fluid will pour out of the vented plug.
I think that you intended to say that you have discs on the front and Drums on the rear.
The 'Valve Cup' in the exploded diagram below is termed the 'Top Hat' seal as when removed looks like a top hat.

Master cylinder 2.jpg (323.87 KiB) Viewed 398 times

This is the non vented filler plug that is required when fitting a remote reservoir:-

https://www.morrisminorspares.com/brake ... it-p829339