Top Hat Seal

[Myrtles Man]

I'm currently looking into the pros/cons whys and wherefores of fitting front discs and, having researched the topic in the archives it's become apparent that it's going to be necessary to remove the top-hat seal from the master cylinder to avoid the problem of the discs binding when the pedal is released. This leaves me slightly puzzled (what? only slightly?!) insofar as the rear drums will be remaining on the car and the following statement (© Philthehill 2016) seems to suggest that retention of the top hat seal will really still be needed for their continued operation:- "...the pressure (8 lb/sq in) is in the drum braked system to ensure that the sealing lips of the cylinder cups are kept expanded against the walls of the brake wheel cylinders. It also assists in conjunction with the top hat seal in the process of bleeding the brakes." Clarification/reassurance (preferably in words of one syllable!) will be greatly appreciated.

[philthehill]

Myrtles Man
You are caught between a rock and a hard place over the residual brake line pressure and top hat seal.

The line pressure (8 lb/sq in) as you say above is required to keep the Minor rear brake cylinder cup seals hard against the brake cylinder walls but that pressure must be removed to allow the disc brake caliper pistons/pads to be pushed away from the discs otherwise the discs will bind and wear not only the pads but the discs themselves.
Again as you say above - Having the top hat seal in place also helps with bleeding the brakes as it restrict the amount of brake fluid returning to the master cylinder.
It is common practice to remove the top hat seal when fitting disc brakes to a Minor (using the Minor master cylinder) and I have not heard of any Minor rear brake cylinder seals failing as a result. That does not mean it could not happen.
The Marina disc brake conversion fitted to my Minor with the standard Minor master cylinder and top hat seal removed never gave any problem over many years with the standard Minor 7" dia rear brakes fitted. I now have Wolseley 1500 8" dia rear brakes fitted which have a different seal arrangement so do not need the residual line pressure. The same Minor master cylinder with top hat seal removed is used.
So to sum up - if you want disc brakes on the front of your Minor you will have to accept that the rear wheel cylinder seals are not pressed hard against the cylinder walls as per factory spec or change the wheel cylinder/brake set up as I have done.

Phil

[simmitc]

I've been driving a Traveller with Marina front discs and Minor rear drums for about 30 years as an every day car; and have never had any problems with the rear brakes, apart from the odd seized cylinder, and that seems to have stopped since I swapped to silicopne fluid.

As an aside, at one time the advice was to retain the top hat seal but drill a hole in it. I did that on a different Traveller and all was well for a few years until one day I was driving along and the car got slower and slower until it literally stopped and stalled. The discs had come on and would release only after the bleed screw had been used to release the pressure. I can suppose only that the drilled hole had somehow closed up. Subsequent removal of the seal removed the problem.

[Myrtles Man]

Thank you gentlemen both. As Dirty Harry famously said, "You've got to ask yourself one question. Do I feel lucky?"

[paul 300358]

You could always fit one of these on the rear brake pipe http://www.mcgillmotorsport.com/brake-r ... rakes-676/ They also do a 2lb one for the front disks. As the others have said, I've never needed one.

[philthehill]

Paul
That is a sensible solution to the lack of residual pressure in the rear brake cylinders when the top hat seal is removed

To enable fitting two of the following will be required:-

http://www.mcgillmotorsport.com/adaptor ... ptor-6635/

As an aside I have just taken delivery of some rose joints from McGill Motorsport and they were very helpful and the joints were reasonably priced.

Phil

[Myrtles Man]

"...to maintain wheel cylinder sealing inside the drums" seems to suggest that, without a pressure-retention device, brake-fluid leakage will occur. I'm taking it that that has not been the experience of those of you already using a top-hat free system whilst still retaining the original drum rears.

[ampwhu]

Another one here has driven around 20 odd years with disc brakes on the front. I removed the top hat seal. Admittedly it was difficult to bleed the brakes at first, but I jacked up each corner in turn and then it became easy. Never had a problem ever.

[philthehill]

As I said above I have never heard of Morris Minor wheel cylinders failing as a result of the top hat seal being removed but that does not mean it will or could not happen.
The risk is low but needs to be considered.

Phil

[paul 300358]

To enable fitting two of the following will be required:-

http://www.mcgillmotorsport.com/adaptor ... ptor-6635/

[/quote]

Thanks for pointing that out, It's one of the things that I intend to do, but only when I have to drain the brake fluid to replace something.