Help! Hard pedal still cant stop!

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redliner
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Help! Hard pedal still cant stop!

Post by redliner » Tue Jul 09, 2013 9:07 pm

I have a D/G coupe with a 68 chevy M/c,39 Ford front drums,57 Olds rear drum adj p-valve,Morrison thru floor pedals.I dint know piston dia. in m/c but have a good pedal and when I push brake it works kinda...but is not optimum....Should I go with a mopar m/c? Should I go smaller piston size than what I have or larger? These are huge drums and should stop better than it does. I have it biased to the rear cuz the front tires are very small...I know...not much info but....do ya think a different m/c would be better?

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Re: Help! Hard pedal still cant stop!

Post by crazyman » Wed Jul 10, 2013 3:57 am

Power?

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Re: Help! Hard pedal still cant stop!

Post by redliner » Wed Jul 10, 2013 5:36 am

crazyman wrote:Power?
no

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Re: Help! Hard pedal still cant stop!

Post by af2 » Thu Jul 11, 2013 9:01 pm

If you do not know the piston Diameter how the heck do you expect any answer?
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Re: Help! Hard pedal still cant stop!

Post by redliner » Thu Jul 11, 2013 10:41 pm

af2 wrote:If you do not know the piston Diameter how the heck do you expect any answer?
Fair enough...silly me...

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Re: Help! Hard pedal still cant stop!

Post by redliner » Sat Jul 13, 2013 1:15 am

af2 wrote:If you do not know the piston Diameter how the heck do you expect any answer?
without pulling it apart,I eyeballed the bore with a tape and it looks like 1-1/16" bore.I believe the pedal ratio is correct so I wonder if a smaller bore m/c will cure my stopping issues? I have the prop valve adjusted so the rear brakes do the stopping...I hope to go with a 9"Ford rear this winter using Strange rear discs...

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Re: Help! Hard pedal still cant stop!

Post by ImportAction » Sat Jul 13, 2013 2:24 am

redliner wrote:
af2 wrote:If you do not know the piston Diameter how the heck do you expect any answer?
without pulling it apart,I eyeballed the bore with a tape and it looks like 1-1/16" bore.I believe the pedal ratio is correct so I wonder if a smaller bore m/c will cure my stopping issues? I have the prop valve adjusted so the rear brakes do the stopping...I hope to go with a 9"Ford rear this winter using Strange rear discs...
If the slave cylinders are in good working order and the pedal ratio is right then it does indeed sound like you need a smaller m/c. If the pedal really is rock hard with 1 1/16 (it's often cast on the outside) I'd go down to 15/16.

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Re: Help! Hard pedal still cant stop!

Post by lada ok » Sun Jul 14, 2013 8:54 pm

redliner wrote:I have a D/G coupe with a 68 chevy M/c,39 Ford front drums,57 Olds rear drum adj p-valve,Morrison thru floor pedals.I dint know piston dia. in m/c but have a good pedal and when I push brake it works kinda...but is not optimum....Should I go with a mopar m/c? Should I go smaller piston size than what I have or larger? These are huge drums and should stop better than it does. I have it biased to the rear cuz the front tires are very small...I know...not much info but....do ya think a different m/c would be better?
ever thought about using an anchor and rope ?

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Re: Help! Hard pedal still cant stop!

Post by Brian P » Sun Jul 14, 2013 9:04 pm

redliner wrote:
af2 wrote:If you do not know the piston Diameter how the heck do you expect any answer?
without pulling it apart,I eyeballed the bore with a tape and it looks like 1-1/16" bore.I believe the pedal ratio is correct so I wonder if a smaller bore m/c will cure my stopping issues? I have the prop valve adjusted so the rear brakes do the stopping...I hope to go with a 9"Ford rear this winter using Strange rear discs...
Really?

By this, do you mean the rear brakes (and the rear brakes ONLY!) are doing the stopping?

If that is the case, that is a problem. I know that drum brakes need a certain threshold pressure to overcome the return springs before they start doing anything, but if you are not getting enough front brake, that is a problem ... a BIG problem.

"39 Ford front drums" and "57 Olds rear drums" tells me nothing; perhaps it tells someone else something, but not me. Small tires or not - you need to get it so that the front brakes lock up just before the rear. A first step might be to get it so that the front brakes do anything at all.

By 1957, GM already knew that the front brakes did most of the work, so their rear brakes were designed to not do as much. It has to do with the wheel cylinder sizes and the mechanical advantage between the wheel cylinder and the brake shoes. You are probably trying to pump a lot of brake line pressure into a set of brakes that were specifically designed to not do much!

And I repeat ... small front tires or not, weight transfer forward happens during braking, so you need good front brakes. Locked front tires (with the rears still turning) will have you sliding and unable to turn, but the front of the car will stay out front, so all you need to do to get steering back is ease up on the brake pedal to get the front wheels turning again. Locked rear tires, particularly at high speed, will send you sideways and spinning.

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Re: Help! Hard pedal still cant stop!

Post by af2 » Sun Jul 14, 2013 11:24 pm

Brian P wrote:
redliner wrote:
af2 wrote:If you do not know the piston Diameter how the heck do you expect any answer?
without pulling it apart,I eyeballed the bore with a tape and it looks like 1-1/16" bore.I believe the pedal ratio is correct so I wonder if a smaller bore m/c will cure my stopping issues? I have the prop valve adjusted so the rear brakes do the stopping...I hope to go with a 9"Ford rear this winter using Strange rear discs...
Really?

By this, do you mean the rear brakes (and the rear brakes ONLY!) are doing the stopping?

If that is the case, that is a problem. I know that drum brakes need a certain threshold pressure to overcome the return springs before they start doing anything, but if you are not getting enough front brake, that is a problem ... a BIG problem.

"39 Ford front drums" and "57 Olds rear drums" tells me nothing; perhaps it tells someone else something, but not me. Small tires or not - you need to get it so that the front brakes lock up just before the rear. A first step might be to get it so that the front brakes do anything at all.

By 1957, GM already knew that the front brakes did most of the work, so their rear brakes were designed to not do as much. It has to do with the wheel cylinder sizes and the mechanical advantage between the wheel cylinder and the brake shoes. You are probably trying to pump a lot of brake line pressure into a set of brakes that were specifically designed to not do much!

And I repeat ... small front tires or not, weight transfer forward happens during braking, so you need good front brakes. Locked front tires (with the rears still turning) will have you sliding and unable to turn, but the front of the car will stay out front, so all you need to do to get steering back is ease up on the brake pedal to get the front wheels turning again. Locked rear tires, particularly at high speed, will send you sideways and spinning.
Brian, Thank you for what I was going to say..
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Re: Help! Hard pedal still cant stop!

Post by redliner » Thu Jul 18, 2013 8:37 pm

Thanks guys for taking the time to help me out! Just the advice I seek. Here is a thought Strange S series rear disc,Speedway motors kit to fit stock type caliper and rotor to my 39 axle,m/c?,adj p-valve,residual valves,Morrison thru floor pedals,3300lb. car ran 124mph in testing,5.00-15sfront,14-32s rear. Ya this thing is very difficult to stop...come to thnk of it....VERY spooky to steer as well! The WORST thing I did was replace the very nice 40 ford box with a used vega unit! mistake! As long as I'm here do they make a good 525 box with a short shaft that doesnt require alot of turns lock to lock? Is the quick ratio something I should stay away from or....There are times when I need to steer very quickly with this Liberty faceplated TKO 600 gearjammer, 550+ h.p. 5.57 spooled 10 second coupe...(with semicrazed greybeard drivin...)

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Re: Help! Hard pedal still cant stop!

Post by Brian P » Tue Aug 06, 2013 5:57 pm

OK, so you are going to 4 wheel disks. Tell us:

Front wheel rolling diameter
Front wheel effective brake disk diameter - not the outside diameter; give me the radius from the wheel centerline to the centerline of the caliper pistons because that is where the disk clamping force is being applied. (An estimate based on the average of the OD of the disk, and the ID of the swept surface of the brake pads, will do.)
Front caliper number of pistons and diameter of pistons
Is the front caliper a floating type with the caliper pistons all on one side, or is it an opposed-piston type with a fixed-in-place housing and half the pistons on each side of it

All of the same for the rear end

What is the front/rear weight distribution of the vehicle
What is the wheelbase
What is the approximate center of gravity height

If you gather up all this information, you might even be able to figure out roughly how much brake line pressure you are going to need, front and rear, and whether it might be appropriate to change the caliper size/arrangement.

You want to rely on having correct brake caliper sizing to get the brake balance as close as possible and minimize reliance on the proportioning valve. If you are not driving the car in winter or on wet, slippery roads, you might want to size the front/rear brake balance for (say) a 0.8 g deceleration with equal tire-to-road friction front and rear, and no proportioning valve. The worst that will happen by doing this, is that IF you find yourself out on a slippery surface, the front end will lock first by a more substantial margin. But as noted in a previous post, from the vehicle stability point of view, this is not a catastrophic situation. And if your car has the typical front-biased weight distribution, the fact that the rear will be a little under-braked on slippery surfaces by using a setup like this, won't really cost you all that much in total braking available. With no proportioning valve, if you want to lock up all four, there's nothing stopping you from doing it ... just tramp on the brake pedal harder. That the fronts will lock first when you do this (if the calipers are sized right), is a GOOD thing.

I have a funny feeling that this calculation will reveal that you need a lot more front brake and less rear than you are proposing, but without numbers, can't prove it.

Also, with the line pressures that I expect that you are going to be needing ... expect to need a power-assist master cylinder, if you don't already have one. There's a reason that every single modern car has power-assist brakes.

With regards to your steering ratio, same idea here. Do the math. Need the data first.
Wheelbase
What's the maximum steering angle left and right of the front wheels
What's the number of turns lock-to-lock of the steering wheel

Most modern cars that have a wheelbase of 100-ish inches have an overall ratio of somewhere near 15:1 between steering wheel rotation, and front wheel steering angle. If it's in that range, the amount that you turn the steering wheel versus the amount that the car changes direction will feel normal. The amount of effort that it takes to turn the wheel is quite another matter and the math is a lot more difficult.

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Re: Help! Hard pedal still cant stop!

Post by redliner » Thu Aug 08, 2013 8:31 pm

Brian P wrote:OK, so you are going to 4 wheel disks. Tell us:

Front wheel rolling diameter
Front wheel effective brake disk diameter - not the outside diameter; give me the radius from the wheel centerline to the centerline of the caliper pistons because that is where the disk clamping force is being applied. (An estimate based on the average of the OD of the disk, and the ID of the swept surface of the brake pads, will do.)
Front caliper number of pistons and diameter of pistons
Is the front caliper a floating type with the caliper pistons all on one side, or is it an opposed-piston type with a fixed-in-place housing and half the pistons on each side of it

All of the same for the rear end

What is the front/rear weight distribution of the vehicle
What is the wheelbase
What is the approximate center of gravity height

If you gather up all this information, you might even be able to figure out roughly how much brake line pressure you are going to need, front and rear, and whether it might be appropriate to change the caliper size/arrangement.

You want to rely on having correct brake caliper sizing to get the brake balance as close as possible and minimize reliance on the proportioning valve. If you are not driving the car in winter or on wet, slippery roads, you might want to size the front/rear brake balance for (say) a 0.8 g deceleration with equal tire-to-road friction front and rear, and no proportioning valve. The worst that will happen by doing this, is that IF you find yourself out on a slippery surface, the front end will lock first by a more substantial margin. But as noted in a previous post, from the vehicle stability point of view, this is not a catastrophic situation. And if your car has the typical front-biased weight distribution, the fact that the rear will be a little under-braked on slippery surfaces by using a setup like this, won't really cost you all that much in total braking available. With no proportioning valve, if you want to lock up all four, there's nothing stopping you from doing it ... just tramp on the brake pedal harder. That the fronts will lock first when you do this (if the calipers are sized right), is a GOOD thing.

I have a funny feeling that this calculation will reveal that you need a lot more front brake and less rear than you are proposing, but without numbers, can't prove it.

Also, with the line pressures that I expect that you are going to be needing ... expect to need a power-assist master cylinder, if you don't already have one. There's a reason that every single modern car has power-assist brakes.

With regards to your steering ratio, same idea here. Do the math. Need the data first.
Wheelbase
What's the maximum steering angle left and right of the front wheels
What's the number of turns lock-to-lock of the steering wheel

Most modern cars that have a wheelbase of 100-ish inches have an overall ratio of somewhere near 15:1 between steering wheel rotation, and front wheel steering angle. If it's in that range, the amount that you turn the steering wheel versus the amount that the car changes direction will feel normal. The amount of effort that it takes to turn the wheel is quite another matter and the math is a lot more difficult.
First off,Thanks for taking the time to help Brian! I will do my homework and get back to ya!!!!

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Re: Help! Hard pedal still cant stop!

Post by Geezer » Thu Oct 31, 2013 6:32 pm

Suggest you invest in a brake psi gauge.
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