Disc brake conversion is still not really "right"

patrick66

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In 2003, I did a disc brake swap from a '73 Duster to my '66 Coronet 500...pretty common then, not so much nowadays! The brakes have always worked, but not to my satisfaction. They've never been really good, just adequate. Here is what I've done since that conversion:

1) Installed a rebuilt power booster and new disc brake dual master cylinder in 2003 when the conversion was done; along with new pads and brake hoses. The calipers from the Duster were in good shape and installed.
2) Five years ago, the LF caliper locked up while I was leaving a car show. Replaced that caliper. Completely replaced the brake fluid at that time with DOT3. Replaced the RF caliper a year later.
3) A few months back, the car took a great amount of effort to stop after just a 15-mile drive! The fluid was low in the front part of the m/cyl, so it was topped off. Later that day, we bled the brakes all the way around. Not much difference. A new master replacement was recommended by a few people, so a new (not r/b) "made in USA" went in.
4) While I was driving the Hot Rod Hundred car run a few weeks back, the brakes once again began to be "flaky". The pedal would go nearly to the floor at times; other times, it was almost normal. I got to a parts store to see if my fluid level was still good and bought brake fluid, just in case. It was normal, but the master cylinder was very hot to the touch...like, just out of an oven hot, but not glowing. The fluid level was normal, however.

So, with this information, I came to the conclusion that perhaps my brake fluid was boiling while I was braking. I'd heard of this, but hadn't experienced it. It was a 220-mile drive, but the moments of using the brakes were few, since we were on State highways for the majority of the run. I've never had a master cylinder be that hot to the touch!

The plan: Check and adjust the adjustment rod in the power booster for proper adjustment; I'm gonna assume for now that it is not. Then, take the car out for a drive around the lake and back home, which is roughly 40 miles at speeds from 35 to 70 mph. If that is not the problem, I'll take the front wheels off and check the discs and the pads for anything out of the ordinary. If things are "normal" again, I guess that's that. If not, I do have new front hoses (still in their bags) that I was going to use on a project that sold several years ago that are the same as the hoses on the Coronet, that I might swap out.

Any ideas? I think I've covered what's up.
 
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Dave P

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Bit of a ramble here.... even if you had a brake issue at the wheel and managed to boil some fluid I really don't see that
transferring all the way back to the master cylinder. Id be more inclined to get an IR temp gun and go for a long drive then shoot
some various parts in the engine compartment and see how hot "hot" actually is. Could be the master is being cooked from
engine heat?
 

patrick66

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Bit of a ramble here.... even if you had a brake issue at the wheel and managed to boil some fluid I really don't see that
transferring all the way back to the master cylinder. Id be more inclined to get an IR temp gun and go for a long drive then shoot
some various parts in the engine compartment and see how hot "hot" actually is. Could be the master is being cooked from
engine heat?

Possibly. Yeah, my description was a little "rambly", I guess, but I figured I'd start from the beginning of the disc installation and go from that point. The hot master cylinder is the first time I had noticed that. It honestly felt like touching a hot exhaust manifold. Does Harbor Freight sell those IR thermometers, I wonder?
 

diesel_lv

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I've had an issue w my front disc conversion in the past. Would have high/tight pedal after initial bleeding, then over time, it would get a little spongy n I'd pump n they'd be good for awhile. Never really saw a difference in fluid level. Installed front n rear residual pressure valves and would still do same thing. Went around car and using a line wrench, broke n retightened each fitting in the entire system about 3 times. This helped considerably. But just recently it started very slowly doing same thing. I ordered the PARKER 2GF-3 Flare Gasket,45 deg.,Copper,Flare that were recommended earlier this week by another member. What I'm thinking is there is such a small leak in a fitting that air gets in but I'm not seeing fluid out.
 

59pwrwgn

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I've had an issue w my front disc conversion in the past. Would have high/tight pedal after initial bleeding, then over time, it would get a little spongy n I'd pump n they'd be good for awhile. Never really saw a difference in fluid level. Installed front n rear residual pressure valves and would still do same thing. Went around car and using a line wrench, broke n retightened each fitting in the entire system about 3 times. This helped considerably. But just recently it started very slowly doing same thing. I ordered the PARKER 2GF-3 Flare Gasket,45 deg.,Copper,Flare that were recommended earlier this week by another member. What I'm thinking is there is such a small leak in a fitting that air gets in but I'm not seeing fluid out.
Did you have manual brakes before. I did a custom conversion in my truck. Power brakes, 13 inch rotor. The truck stopped but I wasn't impressed. While checking the adjustment on the push rod, I realized my pedal ratio was 6 to 1. So I drilled a new hole and change it to 4 to 1, which is a power ratio. 1. Solved the problem.
 

diesel_lv

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Did you have manual brakes before. I did a custom conversion in my truck. Power brakes, 13 inch rotor. The truck stopped but I wasn't impressed. While checking the adjustment on the push rod, I realized my pedal ratio was 6 to 1. So I drilled a new hole and change it to 4 to 1, which is a power ratio. 1. Solved the problem.
I've still got manual disc's.
 

patrick66

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The Coronet had power drums, originally. I ordered an IR thermometer today, should have it by Friday.
 

Mopar Miki

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but the master cylinder was very hot to the touch...like, just out of an oven hot, but not glowing.

the underhood temp is very high, fix the temp problem under the hood
 

Big Bad Dad

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This may not be related, but you may want to check. My Dodge pickup started getting very hot under hood and in front. To the point that touching the hood and top of the fenders was very uncomfortable. WTF was going on? Didn't take long for the transmission to mess up. I found that the transmission cooler in the radiator had stopped up and the transmission was getting overheated. The heat was staying under the hood. I had to do a fluid change and band adjustment. Tried flushing the cooler without success. So I bypassed it and installed an auxillary cooler in front of the radiator. Problem went away. May not be your issue, but pull your dipstick and check your ATF fluid after driving a few miles. It should be pretty warm, but not boiling hot. (My 2cents worth.)
 

patrick66

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but the master cylinder was very hot to the touch...like, just out of an oven hot, but not glowing.

the underhood temp is very high, fix the temp problem under the hood

The car runs fantastic and the temp gauge reads right where it is supposed to. Might see about the trans cooler issue, if there is one. I have a cooler from a 1985 Diplomat cop car that would work well.
 

Sixpactogo

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The Coronet had power drums, originally. I ordered an IR thermometer today, should have it by Friday.
When you installed the rebuilt power brake booster, did you rebuild the original drum booster? That may be your problem from the start. Disc brakes require about twice the boost as drum brakes do. You may need to upgrade to a dual diaphragm booster. I have no input on your heat problem but a lot of people thought they could swap in discs up front and not upgrade the booster. Me included.
 

patrick66

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When you installed the rebuilt power brake booster, did you rebuild the original drum booster? That may be your problem from the start. Disc brakes require about twice the boost as drum brakes do. You may need to upgrade to a dual diaphragm booster. I have no input on your heat problem but a lot of people thought they could swap in discs up front and not upgrade the booster. Me included.

Yes, it was the original booster for the drums. That never even occurred to me...but that makes more sense, since the brakes have never been 100% at all since the conversion. I actually had better braking power with the drums, to be honest.
 

69Bee

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One issue I see is that you should be using DOT4 brake fluid for disc brakes. DOT3 is for Drum/Drum and has a lower boiling point as you are experiencing.
 

Sixpactogo

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Yes, it was the original booster for the drums. That never even occurred to me...but that makes more sense, since the brakes have never been 100% at all since the conversion. I actually had better braking power with the drums, to be honest.
Been there, Done that! I figured some boost was better than none but that is not the case here.
 

Purepony

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Okay so this happened to me a few months back. I don’t know but it’s worth checking, I was driving and my brake light kept coming on, then turn off then on again when I pressed the brakes.

I pull over and the fluid is boiling hot and I see steam so I drive the car home slowly since I still have front brakes

When I get home I realized my wheel seal tore and was letting all the grease inside the shoes on the back so I called dr diff and ordered the green bearings and put those in and all went away.

I’m not saying that’s your problem but i would check just to make sure.

I had never had a master be that hot and it was actually bubbling
 

Big Bad Dad

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Okay so this happened to me a few months back. I don’t know but it’s worth checking, I was driving and my brake light kept coming on, then turn off then on again when I pressed the brakes.

I pull over and the fluid is boiling hot and I see steam so I drive the car home slowly since I still have front brakes

When I get home I realized my wheel seal tore and was letting all the grease inside the shoes on the back so I called dr diff and ordered the green bearings and put those in and all went away.

I’m not saying that’s your problem but i would check just to make sure.

I had never had a master be that hot and it was actually bubbling
How does a grease leak on to the rear brake shoes make the master cylinder get boiling hot?
 

Purepony

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How does a grease leak on to the rear brake shoes make the master cylinder get boiling hot?
I narrowed it down to this. The grease was going on to the shoes and I’m guessing when I hit the brakes it would keep turning because of the grease all over the shoes slipping inside while the wheel cylinder continued to press out

I’m assuming the continues pressure and and drag heat up the fluid and even made it bubble out when I opened the cap.

After the bearings being replaced it hasn’t done it since.

I was freaking out, I was 25-35 miles from home at 7 pm with my brake light flashing on and off everytine I pressed the brakes and a hot master with bubbling fluid pouring out
 

Dan Harris

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I Agree with ‘69 bee, synthetic dot 4 would help by raising boiling point. I had a similar issue on my motor home in the mountains, which was pretty scary. Synthetic dot 4 eliminated the pedal to the floor problem.

Then I’d find the heat source and fix that too.
 

Dan Harris

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I narrowed it down to this. The grease was going on to the shoes and I’m guessing when I hit the brakes it would keep turning because of the grease all over the shoes slipping inside while the wheel cylinder continued to press out

I’m assuming the continues pressure and and drag heat up the fluid and even made it bubble out when I opened the cap.

After the bearings being replaced it hasn’t done it since.

I was freaking out, I was 25-35 miles from home at 7 pm with my brake light flashing on and off everytine I pressed the brakes and a hot master with bubbling fluid pouring out
Need to use “no fling” grease, mopar had a special grease that would not work out around seals. Regular wheel bearing grease will just make a mess. I think Lucas makes a farm implement grease with similar “no fling” properties .
 

RussT Plymouths

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In 2003, I did a disc brake swap from a '73 Duster to my '66 Coronet 500...pretty common then, not so much nowadays!
1) Installed a rebuilt power booster and new disc brake dual master cylinder in 2003 when the conversion was done; along with new pads and brake hoses. The calipers from the Duster were in good shape and installed.
2) Five years ago, the LF caliper locked up while I was leaving a car show. Replaced that caliper. Completely replaced the brake fluid at that time with DOT3. Replaced the RF caliper a year later.

Are you still running the small piston "A" body slider calipers? Going with a slightly larger piston size caliper like the 73 "B" bodies came with could help your braking. Aspen/Volare calipers have larger pistons and may fit without any mods.
 
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