Top hat removal brake master cylinder

[Michiel]

So the top hat is needed to maintain residual pressure for the drum brakes. If I remove it after a disc brake conversion, what will be the effect on the rear drum brakes?

[King Kenny]

I have not noticed any effect on the rear drums in thirty years of doping this modification.

[Bill_qaz]

None that I have noticed in 3 years ownership

[philthehill]

There will be no noticeable effect on the rear drum brakes.
The residual pressure was originally there to keep the wheel cylinder seals against the walls of the wheel cylinder so reducing the possibility of leakage past the seal. In practice with the top hat seal removed the seals in the wheel cylinder do not leak. So the residual pressure was an additional belt and braces solution to a problem that never existed. Over the many years I have had disc brakes on the front of my Minor I have never had any problem with leakage past the wheel cylinder seals.
Some have installed a residual pressure valve into the rear brake line to keep the rear wheel cylinder seals in place but it is not a requirement to do so.

[Michiel]

Ok. So it is not that the rear brakes grip much later than the front disks than with all round drums.

[philthehill]

Nothing to do with the way the brakes operate. Just down to rear wheel cylinder seal management.

[Michiel]

Thanks Phil. Thanks all.

[jaekl]

The function of the residual pressure is also to facilitate system bleeding and more importantly to allow more fluid into the system. When the master piston move past the port of the reservoir, a volume of fluid is moved out into the system which then returns when the pedal is released. Residual pressure in the system results in a vacuum in the master cylinder when the piston returns which is filled by the reservoir when the port is uncovered. Without the residual pressure, it would be like a sealed system and as the pads and linings wear, the pedal would travel more and more. In addition to the need to not have negative pressure in the system to keep out moisture, this is a possible reason the designers include residual pressure mechanism. Without the residual valve disc braked Minors operate satisfactorily because as the fluid cools the reservoir can compensate for lining wear by allowing more fluid into the system. Also there is some delay in the fluid return and with rapid pedal pumping it may be possible for the reservoir to introduce some fluid before the working fluid returns. The system designers can not rely on that. In lieu of the original residual valves it is advisable to add in-line residual valves, 2 psi for discs and 10 psi for drums.

[Bill_qaz]

The recuperation valve in mastercylinder allows fluid back to the internal reservoir. This fluid is pushed back by return springs on drum brake and seal deformation on disc brakes. You don't create a vacuum in lines or master you are moving a fluid volume back and forth. Having no residual pressure valve has no effect on the operation or pedal travel. Phil clearly explained what Morris's engineering reason was for the OE residual pressure valve in his answer earlier in this post.Fitting a residual line pressure valve is unnecessary.

[jaekl]

Press the pedal and X volume of fluid moves our of the MC. It returns. Press it again and X plus a little more goes out due to lining wear goes out again and so on. Eventually the full stroke of the MC is needed for actuation. This especially applies to disc brakes since drums are either manually or automatically adjusted. Generally, the driver doesn't experience this but the designers did have to consider it. With the top hat valve in place the returning fluid moves the internal spring in the MC enough to allow the fluid to flow around the top hat. At some point the spring is stronger than the force exerted my the braking system, so some of the X volume is still in the system but the MC piston has returned home creating a void (vacuum) which is then filled by the reservoir. Now when the pedal is pressed again X volume but not X plus a little bit goes out to do its function. Residual pressure being required to keep the seals against the wheel cylinder walls I think is a misplaced or not a full explanation of a braking system concern. During operation and after use it is desired to have the system at or above ambient pressure so that humidity or moisture can not enter the hydraulic system. Warm operating fluid will cool when the car is parked which will reduce the pressure in the system. Over time the system pressure will be very close to ambient, so what is pushing the seals against the walls? The spring loaded plastic filler in each wheel cylinder. Newer pistons have seals like the secondary MC seal which don't need an assist to maintain the fluid tight seal. Relying on the fluid pressure to create a fluid tight seal is a bit like a conflict of interest. There must be a burst of fluid in order to make the seal so some leaks out?

The automotive (and other) hobby is full of tidbits that are generally accepted and repeated but may only be half true at best. Many times the recommended practice is effective but the rationale is in correct. I came across one recently. On a later model vehicle there is a gasoline vapour system and a common failure is the normally closed valve that purges the vapours. When it fails in the open state, a common symptom is hard starting after filling the fuel tank. The automotive world is split to explain why. One side thinks the vapour displaced when filling the tank floods the engine and thus is hard starting. The other side thinks the open valve creates a vacuum loss in the manifold and thus is hard starting until the line is evacuated again. I lean towards the latter.

[Bill_qaz]

End of discussion

[philthehill]

It does state in BMC Workshop manual Pt No: AKD 530, Section M, General Description - "a small pressure of approximately 8 lb/sq in is maintained in the pipelines to ensure that the cups of the wheel cylinders are kept expanded". This is due to the style of seal.
The style of seal fitted to the Wolseley 1500 and Riley 1.5 does not require the seal (tapered ring style) to have a residual pressure. The fluid is just pushed out of the master cylinder when the pedal is pressed and returned by the brake shoe springs when the pedal is released. Looking at the exploded view of the Wolseley and Riley master cylinder there does not appear to be a recuperation valve and there is no mention of retained line pressure in the respective combined workshop manual which I have.
If you retain the original Morris Minor type cylinders with the cup seals it is totally acceptable to fit a line pressure retaining device for the drum part of the braking system but it is not absolutely necessary as experience shows.
For my part I have Wolseley 1500 8" rear drum brakes and Marina/Ital 9" disc brakes on the front without a servo fitted. There is no residual line pressure in the system.
Even having a small 2 lb/sq in pressure in the disc brake circuit will lead to excessive wear of the pads and discs as the total pressure on the calliper piston will be around 5lb.
The Morris Minor workshop manual does go into great detail of how the valves and seals work in the brake master cylinder but what is important is to understand what works and removing the recuperation valve when fitting disc brakes does work.
Phil

[Bill_qaz]

Maybe of interest
https://share.google/faZvndnJge2cFHK4Z

[jaekl]

Looking at an older version of the workshop manual, the first one I grabbed, I see the statement. The rest of the last paragraph in the general discussion is has some interesting information. There is acknowledgement that there is a delay in the returning fluid, and that when the piston returns before the fluid returns, there is a vacuum that is readily filled by the reservoir. They also felt that there is a lot of flow from the reservoir in both directions. I did get one function correct - The residual pressure aids in bleeding. The spring backed plastic parts in the wheel cylinders are called cup fillers. Perhaps these parts are most useful during assembly to ensure the cup is not misaligned in the cylinder. Looking at the parts books for Riley One Point Five there there is a valve at the pressure end of the MC - valve stem 7H4751 and seal for valve N.S.P.. I don't have a cross section of this MC. I think it isolates the reservoir until there is a void in the MC when the plunger has returned. It may function has a residual pressure valve, but I would need more information.

Thank you for correcting me about the pressure to hold the cups out. I still feel there is a advantage to maintain some pressure in the system when vehicle is in and after use to keep moisture out as the fluid contracts as it cools but I will no longer include it in future discussions..