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Troubleshooting disc brake conversion

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phast1 View Drop Down
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    Posted: Nov/12/2018 at 6:41pm
For the past several months I've had a scarebird large rotor disc brake conversion on the hornet (GM 1/2 ton calipers and Ford sportrac rotors/hubs). The car was a factory manual drum brake car, so I figured I'd try manual disc considering the difficulty of finding small body brake booster assemblies. I first used a CPP universal style master cylinder with an inline residual pressure valve going to the rear drums.

The setup, front bowl of the master cylinder is plumbed to the rear drums
 

I wasn't really satisfied with the braking or pedal feel, and didn't seem like a noticeable improvement over the stock drums. I bled 3 quarts through with the same results. I began to think it was a master cylinder bore sizing issue, and went down to a 15/16 bore. Maybe a slight improvement, but it still wasn't as good as I'd like. After scratching my head for while, I took it to a local shop in hopes that they would see something obvious that I missed, or at least vacuum bleed it as I felt the pedal was still a little soft and maybe there was trapped air somewhere. They told me afterwards that there were no leaks and that brakes were bled, and told me that it's probably a master cylinder sizing issue, but that I should have gone larger bore instead of smaller. Recently I switched to a 1 1/8", and It hasn't made a noticeable difference.

Considering that this was my first disc brake conversion and I don't have a whole lot of experience relating to it, I was wondering if anyone here had any input on where to go from here. 

Connor  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Nov/12/2018 at 10:05pm
i drive manual disc/drum and now disc/disc. stiff as heck and very short travel. they scare people not used to them! i like it just fine. however mine's got 4.5:1 pedal lever ratio, so it's extra stiff. '61 american.

had scarebird+rear drums in a 63 american and they were only slightly stiffer than the previous all manual drums.

had 81? AMC bendix single piston disc fronts and 9x2 drums on the rear of a 63 classic wagon. this and all no booster; manual. on this car they were fairly stiff, and new drivers were initially alarmed, but got used to them very fast as they otherwise worked great (and wasn't a lot of "effort*, it just didn't feel like a typical mushy american car).

i imagine, just guessing, your current beast ought to be closer to my 63 classic; "stiff like an old pickup" but rock-solid and half a day of use "normal".

all of these used various iron OEM type dual circuit master cyls with 1" bores. but i suuuure could use a 15/16" or maybe 7/8" bore on the current beast.

that's not a residual valve in your rear circuit (front M.C. piston) it's an adjustable proportional valve! i think (imho) a better way to adjust front/rear balance -- once you get this working correctly -- is to fiddle with rear wheel cylinder bore. but leave that til last. in fact, since you have it plumbed in, "tighten the screw" to effectively disable the rear brakes, for initial test/setup.

did you BENCH BLEED the master cyl? it takes me 20 - 30 minutes of pressing with a big screwdriver in the piston, against my hip, over and over and over until every single microscopic bubble, the size of a pin POINT, not a pin head, is GONE. ZERO. the tiniest bit of air can be felt. i usedta think i was bad at it when people said "its easy" but it's not. it's tedious as heck and sometimes i have to take it back off, and DO IT AGAIN.

i have some old brake lines that screw intot he ports, bent up and back into the resevoir, below fluid level so that there's no "drop" to make air bubbles as you pump it.

with the rears valved off for now, the fronts ought to fully bind the rotor with an inch or so of pedal travel. maybe 1.5". that's to fullk tire-screeching lockup.


that's a very clean installation you;ve done!

1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Nov/13/2018 at 6:30am
What Tom said! The residual pressure valve in drum brakes holds about 10 psi in the lines to prevent the cup type seals in the wheel cylinders from leaking. It takes around 100 psi just to overcome the springs in the brakes, so no issue there. You have no leaks now, but as the wheel cylinders and seals wear you could. There are in-line RPVs available in 3 psi (for front disc brakes where the MC is lower than the caliper, as in 50s under floor MCs, to prevent fluid drain back) and 10 psi from hot rod brake shops (Speedway Motors has them).

With your adjustable proportioning valve I'd do as Tom suggested -- screw it down and test brakes to see if you have a rear lock-up issue, and if not (all brakes lock at the same time or fronts lock first on a wet or gravel/dirt road -- not dry pavement) you can eliminate the valve. While all 60s and 70s cars have some kind of rear proportioning valve with disc front/drum rear brakes (drum/drum don't), engineers figured out by the 80s (maybe some late 70s cars) that the valve isn't needed with the proper sized drum brakes in the rear -- at least as built from the factory (tires and all). I believe the valves were used early on partly to prevent having two different rear brake setups -- one for front discs, one for front drums. Cheaper and quicker to install the valve and use the same rear brakes on all. Once front discs became standard in the mid 70s some testing could be done and the valves later eliminated. But that's just my theory...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Nov/13/2018 at 6:40am
You don't state your brake pedal ratio. If at some point the car had the pedal replaced from something else, this could be a problem.
Also how much travel do you have until it feels like the brakes engage?
I like a short travel "sharp" brake pedal, like Tom. I made my own pedal rod to raise the pedal which reduces the arc travel and sharpens up the brakes. Personal preference.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote phast1 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/13/2018 at 1:43pm
Originally posted by tomj tomj wrote:

i drive manual disc/drum and now disc/disc. stiff as heck and very short travel. they scare people not used to them! i like it just fine. however mine's got 4.5:1 pedal lever ratio, so it's extra stiff. '61 american.

had scarebird+rear drums in a 63 american and they were only slightly stiffer than the previous all manual drums.

had 81? AMC bendix single piston disc fronts and 9x2 drums on the rear of a 63 classic wagon. this and all no booster; manual. on this car they were fairly stiff, and new drivers were initially alarmed, but got used to them very fast as they otherwise worked great (and wasn't a lot of "effort*, it just didn't feel like a typical mushy american car).

i imagine, just guessing, your current beast ought to be closer to my 63 classic; "stiff like an old pickup" but rock-solid and half a day of use "normal".

all of these used various iron OEM type dual circuit master cyls with 1" bores. but i suuuure could use a 15/16" or maybe 7/8" bore on the current beast.

that's not a residual valve in your rear circuit (front M.C. piston) it's an adjustable proportional valve! i think (imho) a better way to adjust front/rear balance -- once you get this working correctly -- is to fiddle with rear wheel cylinder bore. but leave that til last. in fact, since you have it plumbed in, "tighten the screw" to effectively disable the rear brakes, for initial test/setup.

did you BENCH BLEED the master cyl? it takes me 20 - 30 minutes of pressing with a big screwdriver in the piston, against my hip, over and over and over until every single microscopic bubble, the size of a pin POINT, not a pin head, is GONE. ZERO. the tiniest bit of air can be felt. i usedta think i was bad at it when people said "its easy" but it's not. it's tedious as heck and sometimes i have to take it back off, and DO IT AGAIN.

i have some old brake lines that screw intot he ports, bent up and back into the resevoir, below fluid level so that there's no "drop" to make air bubbles as you pump it.

with the rears valved off for now, the fronts ought to fully bind the rotor with an inch or so of pedal travel. maybe 1.5". that's to fullk tire-screeching lockup.


that's a very clean installation you;ve done!


Thanks! I do have 10 lb inline residual pressure valve, it's just later down the brake line. I'll post a picture later today
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Post Options Post Options   Thanks (0) Thanks(0)   Quote phast1 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/13/2018 at 6:25pm
Here's a pic the of the inline residual pressure valve


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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Nov/13/2018 at 11:41pm
aha! got it. again, nice work!

since you got the prop valve with the knob in there you might as well use it when the time comes.

let us know how diagnosis goes...

1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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