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'49 F4 disc brake conversion, a step backwards

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Old 07-13-2016, 08:18 PM
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'49 F4 disc brake conversion, a step backwards

Well it appears that I didn't do my research well enough. I went to the Ford dealer today to buy the fittings for my master cylinder so I could finish up the brake plumbing. In my brake conversion thread, I mentioned that I am using a master cylinder from a 2012 F450-F550 truck. I found out today, talking to a couple of mechanics, that my dual MC is technically a single MC and will not work for my application. The MC has two ports on the side for lines and it has two ports on top that the plastic reservoir snaps into. The line port on the side closest to the vacuum booster is the pressure port that feeds the ECU and the other port is the return line from the ECU. The MC feeds the ECU and the ECU feeds each brake separately, controlling the brakes for bias and slippage, I.E. computerized ABS. The return line sends the fluid from the ECU back to the MC when you release the brakes. They told me that I would need a MC from a mid '80s or earlier truck and that it would bolt up to my existing booster. So, back to the drawing board.
Mark
 
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Old 07-13-2016, 08:40 PM
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Good thing you found out now!
 
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Old 07-13-2016, 08:51 PM
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You've got that right! Its a good thing that the MC has odd fittings otherwise I would have installed it and would be trying to figure out why it didn't work.
 
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Old 07-17-2016, 11:39 AM
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Mark, Do you know what the diameter of the MC is? I pulled one from an 80's or 90's Ford. I think it says 1 1/8 right on the side of it. It is aluminum with a plastic reservoir on top.
 
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Old 07-17-2016, 08:26 PM
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Jules, The original MC is 1 1/4". I checked out quite a few Ford MCs for F350 and Super Duty trucks. All of them seem to be 1 1/8" or 1". After 1988 they have a pressure port and a return port for ABS, instead of twin pressure ports. With 1 1/2" wheel cylinders on the rear and 60mm dual piston calipers on front, I am not sure a 1 1/8" MC will give me enough fluid volume. I did find a MC from a 1972 Chevy C30 truck that is 1 1/4" bore and has an 11" booster like the MC I was going to use. The C30 had an optional 11,000 rear axle that had 15 x 3 1/2" brakes and 1 1/2" wheel cylinders, which are the same size that my F4 has. It also used 75mm single piston calipers on the front. The C30 system had a proportioning valve which is also available, so since the brakes a so similar to mine, I will go ahead and add one to my system as well, if I can make this MC work.
I started out to rebuild this truck basically original, But now with a Mercedes engine, a Volkswagen back up light switch on the T98, late model F450-F550 disc brakes and now a Chevy MC. Seems like when you start down the slippery slope to the darkside, there is no stopping.
Mark
 
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Old 07-17-2016, 11:39 PM
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Mark, I think the dealer misled you or was confused and you should be fine using the master cylinder you have. I did not have time to look before but my master cylinder is from a 1995 F250 and it certainly has two pressure outputs the forward one is for the front and the rear one takes a small valve that is a combination proportioning valve and residual pressure valve for the drum brakes. Here is an example of the valve that screws into the rear port.

http://www.ebay.com/itm/Ford-Master-Cylinder-Brake-Proportioning-Pressure-Regulator-Control-Valve-OEM-/191569538622?fits=Make%3AFord
on the 1995 Era Ford trucks they had rear abs only for towing I think. There was another valve that mounts on the frame right below the master cylinder that controls your rear brake line pressure based on signals from a sensor in the rear axle that I think senses if the ring gear is turning?
 
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Old 07-18-2016, 09:06 AM
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I'm sorry but those guys are smoking something. There isn't any special return line on the master cylinders. All ABS functions are done internally in the ABS module. I've worked on many of these trucks where they have blown calipers by running the brakes too low and only half of the reservoir empties out.

I'm running a 2001 Expedition master cylinder on my Superduty hydroboost. It's 1 1/4" bore vs the Superduty 1 3/8" bore. I wanted the brake pressure switch to face up rather down.

 
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Old 07-19-2016, 10:04 AM
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I bench bled the MC a few minutes ago to see if what the Ford techs told me was correct, here is what I came up with. Both ports pump pressure. The rear port, the port closest to the booster, pumps first and when it is almost done squirting fluid the front one squirts. The reservoir is not completely divided. The reservoir holds almost two pints of fluid. If you drain one side or the other completely the remaining fluid in the side not drained is only maybe 1/2 cup. So, the rear port is obviously not a return port as I was told. Brake companies sell metering valves so that the rear brakes will apply first, is that the reason the MC works the way it does? Do you think I can use this MC?
Thanks, Mark
 
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Old 07-19-2016, 10:12 AM
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Mark, I would certainly run that master cylinder as it is the correct factory one for your brake calipers. Maybe go back to the dealer and try to order the correct size rear proportioning valve like the one I have linked in my post above.
 
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Old 07-28-2016, 11:44 AM
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I bought some fittings to test the Ford MC that I have to see if the Ford Techs were correct. I ordered 3000psi gauges with the fittings, but they haven't arrived yet. So, I tested it with these 160psi gauges. As you can see both gauges are pegged past the 160psi mark with the pedal depressed. The port for the rear brakes starts to develop pressure before the front port, which is good. It is obvious to me that I don't have a pressure port and a return port as the techs told me. So, I believe I can use this MC. If I install the 3000psi gauges when I get them, what kind of pressure should I expect?
Thanks, Mark
 
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Old 07-28-2016, 11:47 AM
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Mark did you ever get the correct for proportioning valve that goes in the rear port? Not sure on pressure google search says up to 2000psi. You could also calculate it out based on piston diameter and leverage and stuff....
 
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Old 07-28-2016, 12:14 PM
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I believe a forum member posted this information some time ago. I want to give credit to 51dueller. If not him, I do apologize. I've not verified the figures but they seem about what I would expect. I have edited superfluous parts of his post but kept the numbers.


Typical minimum pressure required for a wheel cylinder - 400 PSI
80 lbs of input force on factory 1.25" master cylinder = 400 PSI
If you were to equip a power booster - 800-2000 PSI
(Booster multiplication can be between 2-5 times depending on design but is an unpublished specification)



15/16" wheel cylinder - 276 PSI
1" wheel cylinder - 314 PSI
1 1/16" wheel cylinder - 355 PSI
1 1/18" wheel cylinder - 398 PSI
1 3/16" wheel cylinder - 443 PSI
1 1/4" wheel cylinder - 491 PSI
1 3/8" wheel cylinder - 594 PSI

A word of note is the bigger the wheel cylinder, the longer the pedal travel. So if you make sure you have a proper sized wheel cylinder your brakes will improve, by how much I can't say as there are other factors like adjustment, friction material etc.
 
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Old 07-28-2016, 07:56 PM
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Jules, Thanks for your reply. I haven't checked with Ford on the factory proportioning valve yet. I am not totally convinced that I even need one. The original brakes didn't have or need a prop valve. My thinking, may be flawed, is that the discs will be better than the original drums, so I am already creating more brake bias to the front by adding the discs. My thought also is that if I do use a prop valve, I will probably use a Wilwood adjustable valve, since the MC that I am using is from a truck that had discs front and rear, the factory prop valve may not be set properly for disc-drum?
Thanks again, Mark
 
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Old 07-28-2016, 08:03 PM
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Thank you for that info, Ray. Since I already have the 3000psi gauges coming, I will install them and check the pressure. I will also pick up a booster check valve, vacuum hose and connect it to the vacuum pump on the engine. I will check the pressure with the booster working to see what I will actually have. My vacuum pump has about 8" at an idle and about 15" at about 1/3 throttle. I have a vacuum tank I will install later.
Thanks again, Mark
 
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Old 07-28-2016, 09:50 PM
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The master cylinder you are using was for disk, disk. You will need to put a residual check valve i think about 10# in the rear line for the drum brakes. Then you should be good to go or stop.
Bruce
 
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