Disc Brake Conversion Problem
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New User
Joined: Oct 2009
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From: Carrollton, GA
Disc Brake Conversion Problem
I have a '65 F-100 that I have converted to have discs on all four corners, used the conversion guide from FTE to change the front brakes and found a kit from The Street Machine Fabrication to convert the back. I have bled the brakes, front and back so many times I have lost count. I've bled them with the pedal, have gravity bled them as TSM Fab instructed, I even used a vaccum bleeder, and still have a pedal that travels all the way to the floor. The pedal won't even pump up. It does now rebound about half way, which it didn't do untill I replaced and bench bled the M/C. I noticed in the FTE conversion guide it didn't say whether to use the '74 or '65 M/C pushrod, I used the '65 which is a good bit shorter, and was wondering if the M/C piston wasn't travelling far enough. Does anybody know about the pushrods or have any other sugesstions? New to FTE forums but have used the site before and found good info. Greatly appreciate any help.
Posting Guru
Joined: May 2005
Posts: 1,518
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From: Spokane, WA
I'm assuming this is a manual disc/disc setup with no booster. I run a manual disc/drum system using the shorter pushrod. The later one was way too long, my pedal would hardly move. Another thing to check is make sure all of your bleeders are up. If that checks out o.k., start clamping off brake lines to see if you get a firm pedal. THat may help isolate the problem.
Postmaster
Joined: Apr 2002
Posts: 3,613
Likes: 6
From: Coal country
If your master cylinder is bolted directly to the firewall, the '65 pushrod should be sufficient in length.
The '74 should be a little more than 1 inch longer. You might try using it temporarily to see if that fixes the problem. You can easily shorten it to lower the pedal back down if necessary.
If you are using a power brake booster, you might want to look for one from a '76 or later. It uses a flat bar instead of a pushrod, which makes it easy to re-drill the hole for the brake pedal bolt.
Are you using the correct master cylinder, for a disc/disc setup?
The '74 should be a little more than 1 inch longer. You might try using it temporarily to see if that fixes the problem. You can easily shorten it to lower the pedal back down if necessary.
If you are using a power brake booster, you might want to look for one from a '76 or later. It uses a flat bar instead of a pushrod, which makes it easy to re-drill the hole for the brake pedal bolt.
Are you using the correct master cylinder, for a disc/disc setup?
Posting Guru
Joined: Nov 2003
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From: East Flat Rock NC
'65 HOT TRUCK
Welcome to (FTE)!
What ( proportioning valve ) are you using , if you are using a 73-79 for your front disc swap,
sometimes they kinda get stuck in ( Bypass Mode ) and it does take a few bleedings and it will
switch back! You may want to do a search on ''Proportioning valve reset" .
You may have to reset the valve by hand , I never had to but I have read that others have !
P.S. with you installing rear disc brakes, you may have to install an aftermarket valve,like a wilwood !
Wally Womack
Welcome to (FTE)!
What ( proportioning valve ) are you using , if you are using a 73-79 for your front disc swap,
sometimes they kinda get stuck in ( Bypass Mode ) and it does take a few bleedings and it will
switch back! You may want to do a search on ''Proportioning valve reset" .
You may have to reset the valve by hand , I never had to but I have read that others have !
P.S. with you installing rear disc brakes, you may have to install an aftermarket valve,like a wilwood !
Wally Womack
Last edited by Pro-Street/StateTK; Oct 13, 2009 at 03:52 AM. Reason: Watching my Ps and Qs
Posting Guru
Joined: Jan 2002
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From: Elizabethtown, PA
Rear discs are completely unnecessary, but since you've gone that far...
First, what size line did you use? Is it the same as the original to everything for the full length? The fittings for the master cylinder may be larger on one of the ports but you cannot alter the tubing size from stock or the cylinder will be unable to displace enough fluid to work/bleed the system.
2nd, and also highly likely, is if you are using a stock master cylinder it probably won't work. First, you'll want a 2 chamber system for safety. Second, you'll want the correct bore sizing for a disc/disc setup. A disc/drum master is highly unlikely to work correctly, if at all. The caliper pistons in the rear are going to likely require much more fluid displacement than the little bitty pistons in the original wheel cylinders. As with the line issue, you won't have enough pedal travel to build pressure to bleed the system.
Brake hydraulics rely on the mathematics of fluid displacement. By just tossing on parts *****-nilly you can wind up with brakes that don't work at all, pedals that are far too hard to depress, etc.
First, what size line did you use? Is it the same as the original to everything for the full length? The fittings for the master cylinder may be larger on one of the ports but you cannot alter the tubing size from stock or the cylinder will be unable to displace enough fluid to work/bleed the system.
2nd, and also highly likely, is if you are using a stock master cylinder it probably won't work. First, you'll want a 2 chamber system for safety. Second, you'll want the correct bore sizing for a disc/disc setup. A disc/drum master is highly unlikely to work correctly, if at all. The caliper pistons in the rear are going to likely require much more fluid displacement than the little bitty pistons in the original wheel cylinders. As with the line issue, you won't have enough pedal travel to build pressure to bleed the system.
Brake hydraulics rely on the mathematics of fluid displacement. By just tossing on parts *****-nilly you can wind up with brakes that don't work at all, pedals that are far too hard to depress, etc.
Postmaster
Joined: Apr 2002
Posts: 3,613
Likes: 6
From: Coal country
I think if you answer these questions,we can focus our troubleshooting more accurately: What master cylinder are you using now? From what truck, and year. What proportioning valve are you using? For both front and rear, if not a stock dual valve from a Ford truck. What brake pedal are you using? Stock, and if so, for a manual or automatic transmission, and from what year truck. What other changes from a stock setup are you using? For example, tubing diameter, split rear brake lines, etc.
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New User
Joined: Oct 2009
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From: Carrollton, GA
The master cylinder is a manual reman '74 F-100 from the parts store, the proportioning valve is an aftermarket Summit Racing piece (Summit P/N SUM-G3910). It has a 3-spd manual and the pedal is the stock piece as far as I know. As for the tubing there is 1/4" from the M/C to the proportioning valve, and the rest is 1/8" spliced to the original with flare or ferule couplings and rubber lines and banjo bolts to the calipers. To ddavidv: parts were not thrown on "*****-nilly" this is NOT the first Ford truck that has been converted to 4-wheel discs if you do not believe me go to http://tsmmfg.com/Customer's_Rides.htm and scroll to the bottom of the page, granted, it is an '86. I am a heavy equipment mechanic for a living and troubleshoot hydraulics. Not so sure about the whole fluid displacement thing. Fluid is incompressible the calipers stay filled and ride right on the rotors the movement you feel in the brake pedal is the fraction of an inch between the rotor and pad, the calipers should use less fluid than the old wheel cylinders, that is why you have to use a combo valve on a drum/disc system which includes a sequencing valve to apply the rear brakes before the front, otherwise you will lock up your front brakes well before the rear. Not questioning your intelligence, a brake system may be far different than the hydraulics I am used to, but I am not ignorant.<O
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Senior User
Joined: Jan 2001
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From: taylor usa
65 hot truck, I think ddavidv is correct. If you are using a 74 master cylinder, 74's didn't have 4 wheel discs, then the capacity for rear brakes is insufficient to supply for the long feed to the back down a single line then splitting out to the individual calipers.
Are residual valve needed for this app?
Are residual valve needed for this app?
Posting Guru
Joined: Nov 2003
Posts: 1,230
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From: East Flat Rock NC
Link>>>>>>>>>>>>>>
https://www.ford-trucks.com/forums/8...ing-valve.html
Here is some " General Info" that may help you see you will need the Valve installed to make the
front bakes work right ! I tried one of the "Summit Valves " on mine years ago ,and the front brakes
would not work right ! By the way the " Summit Valves are made by " Wilwood " !
Yes I Know that the link is in (80-86) FORUM !
Link>>>>>>>>>>>>>>>>
https://www.ford-trucks.com/forums/8...ing-valve.html
Wally Womack
https://www.ford-trucks.com/forums/8...ing-valve.html
Here is some " General Info" that may help you see you will need the Valve installed to make the
front bakes work right ! I tried one of the "Summit Valves " on mine years ago ,and the front brakes
would not work right ! By the way the " Summit Valves are made by " Wilwood " !
Yes I Know that the link is in (80-86) FORUM !
Link>>>>>>>>>>>>>>>>
https://www.ford-trucks.com/forums/8...ing-valve.html
Wally Womack
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New User
Joined: Oct 2009
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From: Carrollton, GA
I assume you mean the 73-79 combination valve. Correct? Incidentally I was looking at the troubleshooting section on TSM's website and did mention adding 2# residual valves as a solution to a low pedal, but not a complete lack of brakes. I did back the truck out of the shop to sweep the floor and had a complete lack of brakes, which I would indicate to me that BOTH systems are affected. Could it still be my master cylinder? I'll call TSM and see what they say about the M/C. If they say the M/C is O.K. I guess I'll try the residuals, then the prop valve. I appreciate all the help, maybe I'll know more in a couple days.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com
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P.S. I might try the Coke bottle bleeding first. My grandpa always did something very similar to bleed brakes.<o></o>
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> </o>P.S. I might try the Coke bottle bleeding first. My grandpa always did something very similar to bleed brakes.<o
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Posting Guru
Joined: May 2005
Posts: 1,518
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From: Spokane, WA
I wouldn't want to run residuals because they can actually cause disc brakes to drag. Residuals are designed to keep a small amount of pressure to drum brakes to overcome the return springs. I'd make darn sure all the air is out of there first of all. The Coke bottle method will work fine. A pressure bleeder is the best option.
Cargo Master
Joined: Feb 2004
Posts: 2,045
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From: Central Texas
^^^Once the system is completely full of fluid, line size has nothing to do with it....it could be a 4" line and it still wont have any negative affect. However, I will agree that the calipers in the rear may require more fluid and the MC might not be able to supply enough. But he has no brakes at all which would lead me to believe that the system is not bled, or the pushrod length is not correct and he isn't able to maximize the stroke of the MC...or a combination of a three.