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Master cylinder for TD with disk brakew

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Fsisson Avatar
Fsisson fred sisson
nashville, IN, USA   USA
1938 Morgan 3 Wheeler
1950 MG TD "007"
What master cylinder modifications do you make with a disk brake conversion. Anyone use a dual master cylinder either with disks or drums. What kind & what diameter piston? Proper disk/drum set-up requires removing residual pressure valve from the TD master cylinder and installing 2lb valve in front line and 8-10lb in rear line, yet I don't read of anyone doing that.

As long as Iong as I gotta mess with the master cylinder (hate that job) I might like to use a dual master cylinder for safety at the same time. I know others have done this, just can't find 'em. I can do this... just don't want to spend time reinventing the wheel if I don't have to.

Suggestions appreciated. Thanks .

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LaVerne Avatar
LaVerne LaVerne Downey
Fruita, CO, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
1979 Triumph TR8 "Wedgie"
I have not had any issues with the master cylinder to this point after changing to front discs. Nor have I found a need for a proportioning valve. None of the two seat MG's produced with disc brakes through 1980 used a proportioning valve. There have been a few inventive folks that have fabricated a dual circuit system with non stock master cylinders, but nothing is an off the shelf installation.

Mikelead Mike Leadbeater
York, Yorks, UK   GBR
1953 MG TD
check if the rear wheels lock up first before the fronts, when breaking hard, as this can cause a spin, if so you could fit a restricting valve, my '82 BMW 320 has one to prevent this, as do many cars from this era.

Mike

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Fsisson Avatar
Fsisson fred sisson
nashville, IN, USA   USA
1938 Morgan 3 Wheeler
1950 MG TD "007"
Proportioning valves are a different animal... They vary the pressure differential between the front and rear brakes by various means. There are several designs, all designed to 'balance the braking power between front and rear brakes.

RVP's are 'residual pressure valves' built into OEM master cylinders. The RVP inside of the stock MG TD is an 8lb valve. It maintains a slight pressure (8lbs) in the brake line when you release the brake pedal to keep the shoes in close contact with the drum....

DISK brakes require less residual pressure than drum brakes as drum brakes use a spring to retract the shoes. Disk brakes have no spring to retract the piston therefore they require much less residual pressure to keep the pucks in light contact with the disks (2lbs).

An 8lb RPV (built into the OEM TD master cylinder) provides more pressure on the disks than is recommended . Wilwood recomends removing the internal RPV from the TD master cylinder and installing separate inline RPV valves for the front (disks) and rear (drums).

Aftermarket masters do not have RPV valves built in. It would be nice to find an OEM master cylinder from a car with disk/drums which will have the proper RPV's built in.

From what I read so far, those who have converted to disk front brakes don't bother doing it 'right' as far as residual pressure differential. Maybe 8lbs is OK? Wilwood says it is too much and can heat the disks. Looking for feedback.

LaVerne Avatar
LaVerne LaVerne Downey
Fruita, CO, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
1979 Triumph TR8 "Wedgie"
I discussed this on the MG Enthusiast board and I mentioned, that I have no issue with residual pressure. Your results may differ as there have been at least 5 variations of master cylinders supplied for the TD/TF including at least two by the OEM manufacture. So all I can say with certainty is that the brakes do not drag on my car.

Fsisson Avatar
Fsisson fred sisson
nashville, IN, USA   USA
1938 Morgan 3 Wheeler
1950 MG TD "007"
LaVerne... that is the info I am interested in. Real world experience. Thank you.

That said I can pretty much guarantee that all the variations of the OEM master cylinders you mention have internal residual pressure valves. RPV's are part and parcel of hydraulic brake system design. There is a good description of their function and purpose in the factory TD/TF Workshop Manual.

I'm beginning to suspect that 8lbs of residual pressure, while more than recommended by Wilwood, works OK.

As I stated, the Wilwood instructions recommend... and show how... to remove the RPV valve from the TD master cylinder yet I have not seen that mentioned in any discussion on disk conversions.

I am inclined to try the stock master cylinder like you did & see. I miss driving the car! Meanwhile I'll continue the search for a twin master cylinder that I can adapt at a later date. I like the safety aspect of modern brakes.

Can you guide me to the discussions on the board you mentioned?


Thanks again

LaVerne Avatar
LaVerne LaVerne Downey
Fruita, CO, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
1979 Triumph TR8 "Wedgie"
Go to the MG Enthusiast web site...select the bulletin board....then select the TF-TD forum.....you will find it about half way down the page. You can join the site free of charge. It is generally more active than this site was until the last couple of years. As for the dual circuit master cylinder, you will have to join the site I believe to search the archives. Type in master cylinder and select all of the words. Should bring up a single page and you will have to find it. The fellows name is
JiM Northrupt I think he used a Chevy master cylinder, but it was highly modified.

Declan Burns Avatar
Duesseldorf, NRW, Germany   DEU
When I converted my Morris Minor to disc brakes it was recommended to remove the "top hat" seal (RPV) from the master cylinder. I did not do this and had terrible problems with the brakes binding. The Morris Minor master cylinder is practically identical to the TD/TF. I think the only difference, if at all, is the bore. When I did remove the seal all was well and no more issues with binding brakes. Access to the m/c on a Morris Minor is a nightmare compared to the TD/TF.
Regards
Declan

chuckmosher Avatar
chuckmosher Gold Member Chuck Mosher
Minneapolis, MN, USA   USA
1953 MG TD
1961 Ford Ranchero
1962 MG MGA MkII "Othello"
1968 MG MGC GT "Tordos"
Fred,

Trying to learn some of the lingo here - how does a "proportioning valve" differ from the "Pressure Failure Switch" you see on MGA's and MGB's ?

Here's a picture from my MGC - I have heard this called a "proportioning valve", but sounds like that is a different animal.

Chuck


Attachments:
IMG_0848.jpg    11.5 KB
IMG_0848.jpg

LaVerne Avatar
LaVerne LaVerne Downey
Fruita, CO, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
1979 Triumph TR8 "Wedgie"
A proportioning valve is used to create an ideal balance of braking between the front and rear brakes and is a pretty common item in modern braking systems. The Pressure Warning Switch in your MGC and MGB's as well.. and for that matter one in some form in all cars produced now...is just what it implies. There is a shuttle valve inside the housing that must be centered when the brake system is bled. If you develop a leak in one wheel cylinder and it allows the pressure to be much less on that side of the housing, the shuttle will slide to that side and push the plunger up and complete the electrical circuit causing the warning light on the dash to illuminate. It has nothing in common with a proportioning valve.

Mikelead Mike Leadbeater
York, Yorks, UK   GBR
1953 MG TD
The TD manual states the residual pressure is to keep the wheel cylinder seals seated to prevent air ingress.
Maybe this was a feature of the seals of the day, has anyone experienced this?

Otherwise it seems to me the residual pressure is unnecessary, especially with disc brakes which have no positive pull-of system.

A very interesting discussion.

Mike

Fsisson Avatar
Fsisson fred sisson
nashville, IN, USA   USA
1938 Morgan 3 Wheeler
1950 MG TD "007"
Mike... right... the RPV actually serves a several functions and one is to maintain a slight pressure to keep the seals in contact when the wheel piston retracts.

Another function is in a system where the master cylinder is mounted lower than the disk calipers (as in my TD). Wilwood says there is also a possibility that with no RPV the fluid in the calapers can drain back in time and flood the master cylinder.

All these things are designed into a 'system' by real experts and when the 'system' is messed with... There is a chance of goofing things up. The TD brake 'system' for instance was designed to operate with drums... six wheel cylinders. It operates just fine but... some of us like to mess with things just because... it's fun. But it's real easy to just bolt something on and end up with an unbalanced 'system'.

By the way, I love discussions like this as I learn.

I'm no expert, but over the years I have built several cars with juice brakes where I had to spec/modify/assemble a complete brake system. Still learning.

Example... my 1938 Morgan three wheel club racer... front two wheels changed from original mechanical drums to hydraulics, using British parts.... it has modified/vented Austin Marina backing plates, TR6 vented drums, Spitfire wheel cylinders, clutch 1/2" master, RPV... coupled with the orignal mechanical rear brake using an adjustable progressive rate spring compensater that I thought that I had disigned... I felt like a genius only to be told years later that the old Jaguar SS100's used the same design back in the thirties! Damn... I just reinvented the wheel! Oh well...
I built a gentleman a quite fast Morgan flat-rad that I raced at Put-in-bay a few years ago. I modified the mechanical backing plates to fit twin leading shoe hydraulics using MGB cylinders. Much better. Then I discovered Porterfield shoes. Porterfields are pricey but they are worth every penny. I swear those drums with Porterfield shoes are as effective as disks!
I converted my TD to Wilwood disks because.. I wanted to! Probly could have just ordered Porterfield shoes for the drums, but that's not as much fun....

Mikelead Mike Leadbeater
York, Yorks, UK   GBR
1953 MG TD
Thanks , Fred, interesting stuff, we are all still learning.

Mike

J Stone John R
Aptos, CA, USA   USA
I will also add that Porterfield brake shoes are as good as discs. Very powerful with no fade, like adding a power booster.
Now, a small story about that.

I have Portrfield's shoes on my old F-250 pickup truck and once drove about 100 miles in SF traffic wondering "What is that smell?" Well, you know the answer... I left the parking brake on. The (silicone) fluid boiled out of the rear brakes... The end result was a panic stop with only the front brakes working. I survived, pulled off the road, refilled the master and kept going. After that trip the axle seals leaked (melted) so I disassembled and replaced the seals and springs (paint burned off) but the shoes looked fine so as a test I left them. They are still working as new, 15 years later!

If you can accurately measure your drum ID in inches Porterfield will arc the new shoes to fit.

(Has anyone ever fitted MGA 10" drums to the front of a TD? I already have an MGA rear axle and wheels in the rear)

edit to add; The boiling point of DOT 5 is about 500°f .
The Porterfield shoes never faded and I'm thinking they were probably red hot.



Edited 1 time(s). Last edit at 2019-02-10 04:47 PM by J Stone.

Fsisson Avatar
Fsisson fred sisson
nashville, IN, USA   USA
1938 Morgan 3 Wheeler
1950 MG TD "007"
John.? Agree... radiusing the shoes is a must and Porterfield is the only supplier that I have dealt with that offers that service. Especially with old cars (like TD's...) with drums that have more than likely been turned in the past, radiusing the shoes to the specific drums is a necessity if you want good brakes. It is rare to find a shop today with a machine because of the asbestos dust in old shoes. I've jury-rigged a face plate on my lathe and a belt sander in the past but now I have an old friend who has an old machine.

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