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dodyudynosur1213 Abbas Abboobaker
Durban, kwazulu natal, South Africa   ZAF
1957 Morris 1100 "Miracle"
Hi have just fitted discs in the front and still have drums at rear have heard that I need to modify the master cylinder removal of a seal at the front of the cylinder any help am try to bleed but when the pedal is at top position and when depressed again it squirted from the master is the seals damaged or am I overlooking something will appreciate any help

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emjay Jim English
Marietta, PA, USA   USA
A squirt is expected since the port from the reservoir to cylinder is open when the pedal is fully returned. As the pedal is depressed the initial amount of pressure is shot up into the reservoir. A high squirt means you have air in the system that still was pressurized as the pedal returned. The residual valve or check valve that should be removed really doesn't help or hinder when bleeding since when a bleeder is open, all pressure is removed.

0123 Mike D
Biddulp, Staffs, UK   GBR
If you contact Gerard directly, or wait until he
comes on line he will tell you what you must do Dody.

In the mean time, I cannot see that your bleeding
problem is caused by you not doing those mods.

However, as you need to empty your system,
there's no point in you trying to bleed it now.

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0123 Mike D
Biddulp, Staffs, UK   GBR
To give you some idea of what's involved
here's a picture from the Manual Dody smiling smiley

As I understand it, you need to remove
at least the bit labelled "Valve cup"

I think it is possible to remove it
without taking the master cylinder out.

No doubt Gerard will correct me if I am wrong.



Edited 1 time(s). Last edit at 2017-07-05 12:41 PM by 0123.


Attachments:
Ferd.jpg    38.7 KB
Ferd.jpg

pixelsmithusa Avatar
You are correct about removing the residual valve, but incorrect about removing it without removing the master cylinder. I guess it could be done in situ if you could remove the circlip, pushrod and piston with the MC sitting in the channel, but access is so poor, and the risk of contamination so high, that I personally would not do it that way. I'm not sure you could get the piston out with the pedal in place either. I've never tried.

The residual valve is designed to keep some residual pressure in the lines when the brakes are in a relaxed state. Without it, pressure exerted by the return springs at the brakes shoes will cause the shoes to retract and push fluid back toward the master cylinder. If you were to step on the brake pedal, you'd find travel to be excessive, and a second pump required to bring the shoes up close enough the drum to apply the force needed. You don't want the residual pressure condition to exist with disc brakes as the residual pressure will cause the pads to drag, causing premature wear and potential overheating, which can also increase brake fade. The solution is to either add a restrictor valve internally or use external residual valves. An internal valve is not the best solution due to the master cylinder sitting so low in relation to the brake calipers. The correct solution is to install separate residual valves in the front and rear lines. A 10 PSI valve should be used in the rear line for the drum brakes, and a 2 PSI valve in the front for the discs.

I'm wondering how long it'll take before somebody says "it's not necessary".

In reply to # 24641 by 0123 To give you some idea of what's involved
here's a picture from the Manual Dody smiling smiley

As I understand it, you need to remove
at least the bit labelled "Valve cup"

I think it is possible to remove it
without taking the master cylinder out.

No doubt Gerard will correct me if I am wrong.



Gerard

http://gerardsgarage.com/



Edited 1 time(s). Last edit at 2017-07-05 08:35 PM by pixelsmithusa.

emjay Jim English
Marietta, PA, USA   USA
Gerard, I'm going to throw this out.
I don't think the residue value is to keep the wheel pistons extended. The return springs are strong enough to pull back on 10psi x pi x 7/8sq/4, which is about six pounds. Of course on the big diameter caliper pistons the resulting drag force will be very significant, so your recommended 2psi front residual pressure would yield a lower drag force. But why not zero, then there would be no drag force? I think the reason the original is designed to have a residual is to prevent a vacuum. As the system cools down at rest the fluid pressure could drop below atmospheric creating a vacuum which would allow contaminants to enter the system. With drum brakes with return springs, the design can have a bigger pressure margin but with discs the designs need to be closer to the limit since there are no return springs. But is 2 psi enough? This is where your comment does make sense. Calipers are different than wheel cylinders in that they can retract as the fluid cools and contracts preventing a chance of developing a vacuum. Now having some residual pressure can reduce the amount of pedal travel on a later stroke.

0123 Mike D
Biddulp, Staffs, UK   GBR
What do you consder stops
brake fluid flowing from the
master cylinder past the residual
valve as the fluid cools please Jim?

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emjay Jim English
Marietta, PA, USA   USA
That's right I wanted to mention that since the reservoir is open to the cylinder when the pedal returns and since the cap is vented, a vacuum is not likely, but there is a bit of restriction since the cup valve needs to be deformed a bit to allow the fluid to pass. The best that can be expected is be equal to the atmospheric pressure. The residual valve will provide some positive pressure differential.

Newer systems with sealed reservoirs can develop a vacuum in the system without residual pressure present.

0123 Mike D
Biddulp, Staffs, UK   GBR
What are these newer systems
with sealed reservoirs please Jim?

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0123 Mike D
Biddulp, Staffs, UK   GBR
Are residual pressure valves necessary
if the master cylinder reservoir is higher
than the top of the brake pistons please Gerard?

emjay Jim English
Marietta, PA, USA   USA
Outside of the Minor world there are diaphragms in the reservoir to keep the outside world out. Now that I think about it these diaphragms will still allow pressure equalization with the atmosphere so a significant vacuum is not likely, but maintaining a residual pressure is still a good idea.

emjay Jim English
Marietta, PA, USA   USA
The other thing I wanted to mention. Minors are very digital. The throttle is either closed or often fully open. The brakes are also full or none. The point is the small drum size gives little room for modulation and that is a big thing with newer systems and I think that is what most owners are experiencing with their upgrades. Today we expect good deceleration with a modest pedal knowing that if we push harder it will be even better. With seven inch drums that is all there is. There are many aspects of a braking system and they all have to be in proper balance to make it a good one. If you change one component that will affect another.

geezer Avatar
geezer Silver Member charles durning
Magee, Mississippi, USA   USA
1958 MG Magnette ZB "Chick Magnette (sold)"
1967 Morris Minor 1000 Saloon (2-door) "Marvin"
1974 MG MGB GT
I agree with Gerard.

I did my disc brake conversion in 2 stages. Fronts first and then rear. With the valve removed from the master cylinder the pedal travel was very long. I did as Gerard advised and put the 10lb in the rear and the 2lb in the front. Then the travel was comfortable. When I changed rear to disc I retained the single 2lb inline valve for both front and rear. I am happy and the pedal travel is where I would expect it to be.

No science here just real world experience.



If it was good 60 years ago, does that mean it can't be improved?

0123 Mike D
Biddulp, Staffs, UK   GBR
Where can I find an engine room
based master cylinder please Jim?

dodyudynosur1213 Abbas Abboobaker
Durban, kwazulu natal, South Africa   ZAF
1957 Morris 1100 "Miracle"
Hi thanks guys much appreciated

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