Brakes won’t ‘hold’

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demon322

demon322

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71 Demon. Have done the ‘73 disc brake swap including the disc brake master cylinder. It stops pretty well. I never drove it when it still had the drums so I can’t compare it to that, but it seems to stop just fine.

Earlier this month when I was at the car show/bracket race I noticed the brakes won’t hold the car for me to do a burnout. They hold just long enough to get a few rotations and then the car just pushes forward.

Any ideas of where to check first? Everything *looks* to be in good shape. Plenty of pad left, fluid level good etc.

Thanks.
 
demon322 said:
71 Demon. Have done the ‘73 disc brake swap including the disc brake master cylinder. It stops pretty well. I never drove it when it still had the drums so I can’t compare it to that, but it seems to stop just fine.

Earlier this month when I was at the car show/bracket race I noticed the brakes won’t hold the car for me to do a burnout. They hold just long enough to get a few rotations and then the car just pushes forward.

Any ideas of where to check first? Everything *looks* to be in good shape. Plenty of pad left, fluid level good etc.

Thanks.
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Doing a burnout and the car tends to move forward? [hinkey]
 
Yes. I get that it should move forward. But I should be able to spin enough to get some heat in the tires and a little smoke out the back before it does. My dad in his 68 Satellite could sit there in a cloud of smoke until his tires popped before he moves forward if he wanted to. Something seems off. Like I said, barely a squeak and I’m moving forward.
 
Sounds like the brakes are working great to me.

Power breaking only works on cars with loosely adjusted rear breaks, or a LOT of power......and honestly, you really don't want to spin the rear tires with the rear breaks applied. That's total abuse of the break system.

What I recommend is simply. Pull the car into burnout position. Hold firmly on the break pedal. Raise the RPMs until the car almost pushes through the breaks, then simultaneously STAB the gas to the floor and release the break. You should get lots of tire smoke. What you are doing is loading the converter and making it flash as high as it can.
 
RustyRatRod said:
Sounds like the brakes are working great to me.

Power breaking only works on cars with loosely adjusted rear breaks, or a LOT of power......and honestly, you really don't want to spin the rear tires with the rear breaks applied. That's total abuse of the break system.

What I recommend is simply. Pull the car into burnout position. Hold firmly on the break pedal. Raise the RPMs until the car almost pushes through the breaks, then simultaneously STAB the gas to the floor and release the break. You should get lots of tire smoke. What you are doing is loading the converter and making it flash as high as it can.
Click to expand...
Amen Sister, why abuse your baby that way? I guess you could back off the adjusters but what about installing a line-loc
 
demon322 said:
71 Demon. Have done the ‘73 disc brake swap including the disc brake master cylinder. It stops pretty well. I never drove it when it still had the drums so I can’t compare it to that, but it seems to stop just fine.

Earlier this month when I was at the car show/bracket race I noticed the brakes won’t hold the car for me to do a burnout. They hold just long enough to get a few rotations and then the car just pushes forward.

Any ideas of where to check first? Everything *looks* to be in good shape. Plenty of pad left, fluid level good etc.

Thanks.
Click to expand...

What does the pedal do when this is happening?
I have a line lock for that but I can still easily hold the car with the pedal and light em up on my 73 Dart with front discs and my brakes are not loose in the back.
 
RustyRatRod said:
Sounds like the brakes are working great to me.

Power breaking only works on cars with loosely adjusted rear breaks, or a LOT of power......and honestly, you really don't want to spin the rear tires with the rear breaks applied. That's total abuse of the break system.

What I recommend is simply. Pull the car into burnout position. Hold firmly on the break pedal. Raise the RPMs until the car almost pushes through the breaks, then simultaneously STAB the gas to the floor and release the break. You should get lots of tire smoke. What you are doing is loading the converter and making it flash as high as it can.
Click to expand...
 
I guess I haven’t looked at it this way. I assumed the front brakes did most of the holding/stopping vs the rear. But I could be wrong there for sure.

What you describe is how I’ve been launching the car at the line. Hold the brake. After staged bring rpms up enough to load the rear end and then stab the gas on the last yellow. Leaves pretty decent but I don’t know if it would burnout like this. Just took off. I was only running 12psi in the rears tho ha
 
AdaBada said:
Amen Sister, why abuse your baby that way? I guess you could back off the adjusters but what about installing a line-loc
Click to expand...
I’m certainly not trying to ‘abuse my baby’ as she is most certainly that. Especially not when I only take her to the track a few times a year.

I have actually thought about a line-loc (bc I will be swapping a 5 speed in the not too distant future) but I was concerned that wouldn’t work either if there was a brake issue.

Also I was torn because it will always be at least %95 percent street driven so I didn’t want to see a big red button and wiring in an otherwise pretty stock interior. I may need to get over that though
 
TrailBeast said:
What does the pedal do when this is happening?
I have a line lock for that but I can still easily hold the car with the pedal and light em up on my 73 Dart with front discs and my brakes are not loose in the back.
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It feels pretty solid but as soon as tires start to spin it just lurches forward. Doesn’t really shake or anything. On the street with my normal 30ish psi in the tears it’s a similar tale. They start to spin but roll forward pretty quick. Can’t sit still and do a burnout, but can leave about a 9 or 10 ft pair of black marks.
 
demon322 said:
I guess I haven’t looked at it this way. I assumed the front brakes did most of the holding/stopping vs the rear. But I could be wrong there for sure.

What you describe is how I’ve been launching the car at the line. Hold the brake. After staged bring rpms up enough to load the rear end and then stab the gas on the last yellow. Leaves pretty decent but I don’t know if it would burnout like this. Just took off. I was only running 12psi in the rears tho ha
Click to expand...

They certainly do, you're right. But that doesn't mean the rear does nothing. I would say it's about a 70/30 split. Still though, it's a terrible idea to do. Yes, LOTS of people do and have done it, but I have actually seen rear shoe material liquefy and drool off the shoe frames. That's a nasty mess that I promise you do not want.

Besides the line lock, another way to do this is putting an adjustable valve in the rear brake line like the Mopar Performance adjustable proportioning valve. I have even seen people rout in a simple cutoff valve to the rear lines and put it right under the driver's seat. Just some ideas.
 
Yes - hot spot the rear drums and wear down the shoe linings. Does not make for a good street braking system; less safe. Its not easy making a car into a jack-of-all trades.
 
demon322 said:
I’m certainly not trying to ‘abuse my baby’ as she is most certainly that. Especially not when I only take her to the track a few times a year.

I have actually thought about a line-loc (bc I will be swapping a 5 speed in the not too distant future) but I was concerned that wouldn’t work either if there was a brake issue.

Also I was torn because it will always be at least %95 percent street driven so I didn’t want to see a big red button and wiring in an otherwise pretty stock interior. I may need to get over that though
Click to expand...
I set my line lock up with a 3 way switch using the output side of the horn relay.
switch up, horn. switch down, line lock.
pull thru bleach box, set brakes hard, hold horn button. release brakes, apply gas, release horn button when you see smoke. easy and "invisible"!!
 
What proporting valve did you use for your disc conversion?
 
demon322 said:
I guess I haven’t looked at it this way. I assumed the front brakes did most of the holding/stopping vs the rear. But I could be wrong there for sure.

What you describe is how I’ve been launching the car at the line. Hold the brake. After staged bring rpms up enough to load the rear end and then stab the gas on the last yellow. Leaves pretty decent but I don’t know if it would burnout like this. Just took off. I was only running 12psi in the rears tho ha
Click to expand...

Gotta say it “sounds” like you’ve got enough heat for tires to work. If it’s launching hard without spinning. Do as RRR said and let it pull itself out. Maybe not sit in water box but roll thru get some water on tires and do your burnout just forward of the box. As for Smokey burnout, some think it’s cool but if a tire and suspension are working you may only get a few revolutions on the tire before it bites and launches. To me beyond that you need a line lock or other to lower the bias of rear brakes. The issue with adjustable proportioning valve is if you adjust for burnout you’ve gotta readjust back so you have proper bias for normal braking.
 
General Disarray said:
Its not easy making a car into a jack-of-all trades.
Click to expand...
Very true and as stated the plan was always street car that I occasionally take to the track, not the other way around. I have to remind myself of that sometimes ha. The track is a blast, but she’s a street car, not a race car.
 
motorpirate said:
I set my line lock up with a 3 way switch using the output side of the horn relay.
switch up, horn. switch down, line lock.
pull thru bleach box, set brakes hard, hold horn button. release brakes, apply gas, release horn button when you see smoke. easy and "invisible"!!
Click to expand...
I LOVE this idea. If I decide to go with the line lock I would love to have this setup.
 
toolmanmike said:
What proporting valve did you use for your disc conversion?
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I got it from the guy I bought the disc swap from. He said it was for the disc cars but I can’t say for certain that’s correct

EC09E5BB-950C-4123-8957-05E6987E8BE8.jpeg
 
Dubob said:
Gotta say it “sounds” like you’ve got enough heat for tires to work. If it’s launching hard without spinning. Do as RRR said and let it pull itself out. Maybe not sit in water box but roll thru get some water on tires and do your burnout just forward of the box. As for Smokey burnout, some think it’s cool but if a tire and suspension are working you may only get a few revolutions on the tire before it bites and launches. To me beyond that you need a line lock or other to lower the bias of rear brakes. The issue with adjustable proportioning valve is if you adjust for burnout you’ve gotta readjust back so you have proper bias for normal braking.
Click to expand...
That’s what it does at launch. A quick squeak and squat and it’s off. My best 60ft was 1.99. I like the line lock idea more than adjusting the rear bias because as you mentioned it just has to be adjusted back.
 
demon322 said:
I got it from the guy I bought the disc swap from. He said it was for the disc cars but I can’t say for certain that’s correct

View attachment 1715195340
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Drum brake cars do not have a proportioning valve, only a distribution block. What you have pictured is a proportioning valve, so it is correct.
 
RustyRatRod said:
Drum brake cars do not have a proportioning valve, only a distribution block. What you have pictured is a proportioning valve, so it is correct.
Click to expand...

Absolutely.

Still could be a proportioning problem though. Basically what's happening is you're partially applying the brakes. Apply the brakes too hard and you can't spin the rear wheels, apply them too lightly and you don't have enough braking force to hold the car in place. What you need is a larger proportioning difference between the front and rear brakes, so you have less force on the rear brakes when you have enough front brake to hold the car.

The stock proportioning valve was designed to stop the car the best, not do the best burnouts. An adjustable proportioning valve would let you dial back the force of the rear brakes, so you could step on the brakes harder and still spin the rear wheels. Now, that being said, you wouldn't stop as well the rest of the time, assuming your car isn't currently set up with too much rear brake bias.
 
72bluNblu said:
Absolutely.

Still could be a proportioning problem though. Basically what's happening is you're partially applying the brakes. Apply the brakes too hard and you can't spin the rear wheels, apply them too lightly and you don't have enough braking force to hold the car in place. What you need is a larger proportioning difference between the front and rear brakes, so you have less force on the rear brakes when you have enough front brake to hold the car.

The stock proportioning valve was designed to stop the car the best, not do the best burnouts. An adjustable proportioning valve would let you dial back the force of the rear brakes, so you could step on the brakes harder and still spin the rear wheels. Now, that being said, you wouldn't stop as well the rest of the time, assuming your car isn't currently set up with too much rear brake bias.
Click to expand...
As in there could be something wrong with my proportioning valve? Or just the way the system is designed. I’m
Starting to think since everything else is working and the car is stopping I probably shouldn’t worry too much with this.

I do like the line lock setup mentioned earlier. Might that be something worthwhile to avoid the bias problem by locking the fronts only?

Thanks all for the reply’s
 
demon322 said:
As in there could be something wrong with my proportioning valve? Or just the way the system is designed. I’m
Starting to think since everything else is working and the car is stopping I probably shouldn’t worry too much with this.

I do like the line lock setup mentioned earlier. Might that be something worthwhile to avoid the bias problem by locking the fronts only?

Thanks all for the reply’s
Click to expand...

Yeah I don’t think there’s anything wrong with that proportioning valve other than it’s not designed for brake stands.

The line lock is the way to go. If you add an adjustable proportioning valve to reduce the rear braking power it will mess with how the car stops too, the line lock won’t and it will do what you need for the burn out.
 
72bluNblu said:
Yeah I don’t think there’s anything wrong with that proportioning valve other than it’s not designed for brake stands.

The line lock is the way to go. If you add an adjustable proportioning valve to reduce the rear braking power it will mess with how the car stops too, the line lock won’t and it will do what you need for the burn out.
Click to expand...
Sounds good I need to look into that further for sure before the next track trip. I’ll do some more research on them.

What about when (if) I do the 5 speed swap in the future? What do people use there for burn outs and launches?
 
I don't think you have brake issues either. Don't want to get you chasing a problem you don't have. Sounds like everything is working well. My suggestion would be to get the car tuned very well. That way, you can probably do a burnout simply by stabbing the gas pedal.
 
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