Master Cylinder for Disc/Disc conversion?

Knebel

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What kind of master cylinder would I need for a disc setup front and rear? Will I need a residual pressure valve for the rear and a proportioning valve too?

I have manual drums and will keep it manual just with discs all around. Got a very good deal where the rear brake was essentially free!

68 fury iii
 

Davea Lux

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What kind of master cylinder would I need for a disc setup front and rear? Will I need a residual pressure valve for the rear and a proportioning valve too?

I have manual drums and will keep it manual just with discs all around. Got a very good deal where the rear brake was essentially free!

68 fury iii

You need a power assist if you are going to run 4 wheel discs. I takes more hydraulic pressure to activate the discs than you can apply with your foot. I would suggest going to a late model master cylinder and booster. It is important to use a large volume master cylinder with equally sized fluid pots for the front and rear to compensate for pad wear. You will need a proportioning valve to balance the system. You will probably also need larger diameter brake tubing to the discs on the rear as discs take a larger fluid push to operate properly.

Dave
 

Knebel

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Dang. Okay i will need a vaccum pump then probably. My engine pulls about 12"HG.

I have not looked into this too much so I have no clue what booster and master.
 

lemondana

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Also-every rear disc brake kit that I've seen mandates using the non adjustable Green axle bearings, Yuk! Your choice?
 

Justin Plant

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you might look into a hydro-boost unit. They are everywhere in junk yards. GM uses the hell out of them in their trucks. Not going to be easy, but worth a look
 

Ross Wooldridge

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Respectfully - why consider rear discs at all? What are the advantages? As far as I have seen, the advantages of rear discs are negligeable over rear drums in a street driven car.

A front disc conversion alone will be much easier, since if you go with rear discs you will have to not only deal with those issues, but you'll need to convert to a power brake setup, which will entail a new brake pedal assembly (power and manual brake pedals are different), getting the correct booster, etc. etc.
 

69PHOENIX

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Two Points!
I Agree with Ross, IMO There Really is Not a Lot of Advantage to Rear Discs Unless You're a Crazy Driver. Remember 70% of your Braking is Done by the Front Brakes.
If Anything You Might Look at Fitting a Set of the Rear Backing Plates, Shoes & Drums from a 2 1/2 " Set Up if your Running the 2" Ones.
Regards the Disc Setups, a Few Years Ago I Noticed that There was a Disc Conversion Available that used the Tapered Bearings.
I Noticed This Because I was Cussin' the Fact I had Already Bought the Green Bearing Setup,
Anyway Just My Thoughts.
Tony.M
(We have Done a Number of Conversions on the 65-68 Fury's [Dodge Phoenix] using the 69-72 Model Gear.
The fellas with 4 Wheel Drums Love the Front Disc Conversion, But I've been Told Not to Expect any Dramatic Improvement by adding Rear Discs to a Disc / Drum Setup.)
 

Knebel

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I got a very good deal on the kit. Essentially got the rear disc brake for free. I would like to use them and I have read a few things that a 15/16 master would work just fine to keep them manual. I have looked into the hydraboost a little bit but i need to educate myself more...
 

Knebel

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Okay I have ordered a disc/disc master with booster and an electric vacuum pump.

A question, in order to put the rear brakes on, the instructions call for the axle to be pulled out, the drum backing plate removed and a spacer installed in its place. I have no space to really pull the axles and was gonna just cut the backingplate and remove it, its trash anyway. Question about that spacer, would it hurt to cut it and have the round spacer in the pic a 2 part item just like the bracket that holds the caliper? What would you think?

20220514_132340~2.jpg
 

Davea Lux

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You should get the car out someplace where you have room to pull the axles. Cutting the spacer is not a good idea as it would be likely to leak.

Dave
 

Knebel

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You should get the car out someplace where you have room to pull the axles. Cutting the spacer is not a good idea as it would be likely to leak.

Dave
Thats what I though. I will figure it out, I should have JUST enough room for it to do one side at a time. Think... one car garage.

Also, how annoying, there are new wheel lug bolts included that are just a tad longer but they are all RH. Of course i have the rh/lh lugnuts and just bough new lugnuts with my rims. Other than that, it seems like a pretty easy install. Came with new brakelines too!
 

Knebel

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Got the rears on. I'm fighting with the E brake. I loosened the splitter in the back all the way and the drivers side went on no issue but on the passenger side it does not seem long enough. Is there another adjustment that I am not seeing?
 

fury fan

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You need a power assist if you are going to run 4 wheel discs. I takes more hydraulic pressure to activate the discs than you can apply with your foot. I would suggest going to a late model master cylinder and booster. It is important to use a large volume master cylinder with equally sized fluid pots for the front and rear to compensate for pad wear. You will need a proportioning valve to balance the system. You will probably also need larger diameter brake tubing to the discs on the rear as discs take a larger fluid push to operate properly.

Dave
I disagree that you need larger tubing to the rear. Chrysler didn't use larger tubing to front discs vs front drums. And P1V1=P2V2, so a larger tube doesn't improve performance if the MC is appropriate.

I can't speak for 4-wheel discs, but for front discs you absolutely do *not* need a power booster.
I converted my 68 Fury to 73-style discs, used the common aftermarket combination valve, and put the 'factory' MC on my manual-brake pedal.
MC is from mid-70s Dodge truck with manual discs. (I've never seen such a truck, but parts listings show it was available.)
I bedded the pads per some internet method, and I have decent 235-60-15 tires - it stops very well. Pedal feel isn't noticeably different than it was with manual drums, and I like the feel and connectivity between foot and the road. If I was converting another car, and had both MB and PB pedals to choose from - I likely would pick the MB.

Everything from here onward is my engineering thoughts arising from a 15-min consideration - so it's a rough draft - anyone reading must verify for themselves before installing to their car and 'publishing' it to a public roadway:

Rear brakes are not doing much braking vs the front, but you need to ensure enough fluid capacity. Once a suitable boresize is selected, the MC must be reviewed for 2 things - enough fluid volume to activate the brakes, and enough reserve volume to keep the piston submergered as the linings wear and the circuit becomes larger. You could compare this by calculating fluid volume of the wheel cylinders vs the calipers. This isn't perfect, as the friction lining wear must be taken into account - easy to calculate that into extra fluid volume on the caliper piston, a little complex on the drum brake as it might not be a 1:1 of lining wear vs wheel cyl piston extension? You probably could do it by simply extending a wheel cyl to its max and measuring full extension. Another option would be to get a MC and fill the chamber with brake fluid, then pour into a graduated cup.

To check your work:
Fill the rear circuit to capacity with the calipers/ and new pads NOT clamped to the rotor but in 'idle' position. Then press the brake to extend the pistons/pads and review the remaining fluid volume. The difference in volume is what the reservoir must handle in normal operation. Then you could remove the rear pads, install some appropriate shims to emulate worn pads, and see how much further the fluid level drops.

And if there is sufficient volume for operation, but gets borderline when pads wear to 75% or so - you could always top off the fluid level during a periodic maintenance check?
 
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I installed a Leed brakes rear disk kit (single piston floating caliper) because I had serious brake fade problems at the bottom of a twisty hill. I also wanted a more linear application of the brake pressure, the drums seemed excessively prone to locking. I found the pedal feel to be unacceptably soft with the stock MC and the brakes as a whole didn't stop very hard. But no fade!

The truth is that disk brakes take more fluid volume and pressure to work. My choice of solution was to go for a disk/disk MC, I just finished installing a Baer Remaster with adjustable proportioning valve. I still need to bleed the brakes and test drive (a fence project this weekend is getting in the way of that), so I don't know if my problem is solved or if it is just a more expensive version of the same problem... I'll let you know!

Regarding manual brakes, you will need to order a MC with a smaller diameter piston, but they should work just fine. F1 cars use manual brakes!
mc.jpg
 

Davea Lux

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I disagree that you need larger tubing to the rear. Chrysler didn't use larger tubing to front discs vs front drums. And P1V1=P2V2, so a larger tube doesn't improve performance if the MC is appropriate.

I can't speak for 4-wheel discs, but for front discs you absolutely do *not* need a power booster.
I converted my 68 Fury to 73-style discs, used the common aftermarket combination valve, and put the 'factory' MC on my manual-brake pedal.
MC is from mid-70s Dodge truck with manual discs. (I've never seen such a truck, but parts listings show it was available.)
I bedded the pads per some internet method, and I have decent 235-60-15 tires - it stops very well. Pedal feel isn't noticeably different than it was with manual drums, and I like the feel and connectivity between foot and the road. If I was converting another car, and had both MB and PB pedals to choose from - I likely would pick the MB.

Everything from here onward is my engineering thoughts arising from a 15-min consideration - so it's a rough draft - anyone reading must verify for themselves before installing to their car and 'publishing' it to a public roadway:

Rear brakes are not doing much braking vs the front, but you need to ensure enough fluid capacity. Once a suitable boresize is selected, the MC must be reviewed for 2 things - enough fluid volume to activate the brakes, and enough reserve volume to keep the piston submergered as the linings wear and the circuit becomes larger. You could compare this by calculating fluid volume of the wheel cylinders vs the calipers. This isn't perfect, as the friction lining wear must be taken into account - easy to calculate that into extra fluid volume on the caliper piston, a little complex on the drum brake as it might not be a 1:1 of lining wear vs wheel cyl piston extension? You probably could do it by simply extending a wheel cyl to its max and measuring full extension. Another option would be to get a MC and fill the chamber with brake fluid, then pour into a graduated cup.

To check your work:
Fill the rear circuit to capacity with the calipers/ and new pads NOT clamped to the rotor but in 'idle' position. Then press the brake to extend the pistons/pads and review the remaining fluid volume. The difference in volume is what the reservoir must handle in normal operation. Then you could remove the rear pads, install some appropriate shims to emulate worn pads, and see how much further the fluid level drops.

And if there is sufficient volume for operation, but gets borderline when pads wear to 75% or so - you could always top off the fluid level during a periodic maintenance check?

Factory F/R disc setups run fluid chambers of equal volume to compensate for pad wear. Adding fluid to compensate for pad wear will result in the fluid chamber being over filled when the caliper pistons are retracted to replace the pads. If the system is flushed with the pad replacement this won't be an issue but a lot of folks just throw on pads and put the car back on the road. Chrysler went with a power assist on all C-Bodies, so I would trust their engineering on that. Yes, it will still stop without the power assist, just not as fast. If the purpose of the disk conversion was to improve braking, I would suggest that a power assist would be ideal, but if you find manual braking satisfactory, then go with it. Tubing size on an after market conversion should be sized to the manufacturers specs, hence, a different sized tubing MIGHT be necessary.

Dave
 
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