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Brake info needed.


JACK M
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OK older than 25 years however not stock. 

Just looking for some educated info.

So, forgive me.

I have this highly modified 28 Dodge.

It has a 392 and a four speed.

Problem is the brakes are weak.

I am using a 9" with disc rear brakes kit that uses calipers from a early GM K body (Cadillac Seville). 

The front is basically a stock C4 Corvette. The C4 brakes are a two piston type.

The booster is one of those smaller under floor jobs (I think 7" dual chamber) and I have 16 inches of vacuum.

Master cylinder has a one inch bore. Its a cast iron GM type.

I have the disc disc combination valve in place and 2lb Residual valves front and rear.

I put this together several years ago and have never liked the brakes. Just ok if I don't have to make a panic stop.

I read somewhere that the C4 used a 7/8" bore M/C.  (HMMMM???)

I have plenty if pedal but it takes a lot of leg to get it stopped, especially if its a hurry up stop.

So I am thinking that a smaller bore M/C would make more pressure with the same leg effort..

For the life of me I cannot find a cast GM M/C for sale anywhere in a 7/8' bore.

Not all that long ago I can swear that I found one by application but it had plastic reservoirs, I might be able to live with that but would probably have to modify the floor board if its tall.

Or coarse I cant even find that one now let alone a cast iron disc disc  in a 7/8".

Sorry about the ramble but tell me your thoughts.

Or chastise me IDC,,,,,,(need brakes)

 

28 Dodge project 027.jpg

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28 Dodge project 001.jpg

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Define "weak". To be honest. the best thing to do would be to get a brake line pressure gauge and figure out exactly what line pressure you have.

Does the system have a proportioning valve? If so, it IS plumbed to the back brakes, right? Fixed or adjustable?

Yes, the smaller the master cylinder bore, the higher the line pressure for a given pedal pressure, but before going that far, what is the pedal ratio? That also has a huge effect on line pressure vs pedal pressure.

 

calculating-power-pedal-ratio.jpg

 

CPP sells a Corvette style dual M/C with 15/16" bore (P/N MC1516L).

 

https://www.ebay.com/p/1822094560

 

Also, your dual 7" booster is pretty marginal, especially if your engine has low vacuum. If you can't fit a larger booster, consider going with a hydraboost.

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Thanks Joe,

Weak meaning I can stand hard on the pedal and only slow down. No way I could ever lock these up unless I was on wet grass. No separate proportioning valve, but a combination valve supposedly designed for this application. This one,   https://www.mpbrakes.com/pc_combined_results.asp?search_prod=(searchlike~p.sku~vl3360k|Or|searchlike~ p.nm~vl3360k|Or|searchlike~ p.ds~vl3360k|Or|searchlike~ p.search_terms~vl3360k|Or|searchlike~ p.child_rollup_search_terms~vl3360k)&search_keyword=vl3360k 

It shows as a disc drum however MPBrakes assures me that this valve assy that I have has been modified to disc disc.

This kit has a pedal that looks just like your image on the right. I will measure the ratio tomorrow to be sure 

One test last night was to put a pressure gauge by removing the LF bleeder.

As the car is fenderless and a few feet up on the lift I could start the car and see the gauge standing next to the driver door. I could push on the brake pedal with all my might with my right arm. Most likely that would be casual braking as my leg would be somewhat stronger.

I got a hair over 500 lbs. 

I don't think that is enough pressure.

This was supposed to be an engineered kit, it came with the under floor booster and master cyl. along with the modified combination valve.

I also ordered the rear brake kit from them at the same time. I stepped up pretty big on this deal, when I put this thing together and was disappointed.

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Here are a few things to think about:

 

Assuming manual brakes or a conventional booster:

 

1) The area of the master cylinder vs the area of the calipers is calculable. You need to be able to produce enough pressure, and this does indeed calculate out in pounds per square inch. A smaller bore gives you more pressure and easier braking, but more travel. Systems are designed around a certain amount of "push" at the pedal. What is considered normal today is a bit soft by 1970s standards, but it hasn't changed that much. Sorry I cant quote numbers. I have done a bunch of this but it was 40 years ago.

 

2) If a power booster is used, the diaphragm area vs the vacuum available is also calculable. There is a limit to how much it can help you push. Get this wrong, and you can easily make something that has normal "one toe" braking until you have to panic stop. Then you may have to stand on it with both feet because the needed pressure is still rising but the booster is already doing all it can. Not ideal.

 

3) If you try to fix this with a smaller cylinder bore, the travel increases, but the travel of a brake booster is limited. Some are as little as 1-1/4". The combination of needing enough power while keeping the travel short enough for the brake booster imposes a limit on pedal ratio, In practice, it needs to be within a *very narrow* range. Joe's post has 4:1, and that must be right although I admit I can't remember.

 

4) You need more travel for a dual master cylinder. Most disc systems will need a dual cylinder anyway for other reasons. To take advantage of the "safety" a dual master cylinder offers, you have to be able to bottom the cylinder out mechanically (if the cylinder can take it) or almost bottom it (if the cylinder is not designed to be bottomed). The reason is that there is a "fluid piston" inbetween the pistons in a dual master cylinder, and if you lose half the brakes, you also lose some pedal travel. This is the reason many dual master cylinder conversions cannot take advantage of the alleged "safety". They don't have enough travel to do it. I have also seen factory systems that probably can't do it. That doesn't apply to you since you are building from scratch, but im throwing it out there so you won't un-knowingly make that mistake. This is another reason you don't really have any leeway on pedal ratio with a booster, It has to be right.

 

Once again, these comments apply to conventional vacuum boosters driving conventional master cylinders.

 

5) If you make the brakes manual, there is also a default pedal ratio that is normal. It is higher than the one for power brakes. I can't remember that one offhand either. Maybe Joe has it. You have a little more leeway with manual brakes, substituting pedal ratio for master cylinder piston size to develop the necessary pressure. Only a little more though. You need to not run out of travel in the cylinder for one thing.

 

6) Floor pedals with the cylinder under the floor are more likely to work out with manual brakes than power. As the ratio gets smaller, the cylinder (and the booster if there is one) move UP. The brake booster probably wants to live in the same spot as your feet in the footwell.

 

There are more booster options than there used to be, but you are stuck with the "power" pedal ratio, and you need enough diaphragm area to do the job, and all of this is not only bigger around, but wants to be higher in the car. In a few cars this can work correctly. In most cars it never will. There isn't room. This is one reason almost all factory power brake setups have firewall mounted master cylinders on a reinforced firewall with hanging pedals.

 

The factory power brake systems that DO have the master cylinder under the floor usually use a remote booster (hydravac or similar). I wouldn't rule it out.

 

I have seen pedal assemblies online marketed toward rodders that have some ludricous long pedal connected to a booster that is supposed to go under the floor. I have seen several of these advertised that cannot ever work properly. The pedal ratio is all wrong, and you can see it and measure it right on the computer screen.

 

7) You can get around pedal ratio problems to some extent with a ratio rod, a linkage to change the ratio in less space. Some factory systems have one, as do some aftermarket street rod parts. You could even have more than one, but one is probably almost too many. You definitely want to get rid of as much slop as possible because pedal travel is one of your main limiting factors. Linkages add slop. You could probably solve most problems with about 3 ratio rods, but it adds so much slop it is impractical. It wears out fast too.

 

Sorry I don't have any solid numbers to throw at you. It has just been too long.

 

Edited by Bloo (see edit history)
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Mike, I sold that truck several years ago. Fun stuff...

I had to replace one head gasket to get it running.

It ran ok, and believe me, there were no mosquitos in the neighborhood whenever I drove it.

IM002870.JPG

Fire truck 023.jpg

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Thanks Bloo.

I am going to have to read that again in the morning.    LOL

It would be difficult to change the pedal ratio in this case.

This system was supposed to be already engineered.

I am also going to see just how much trouble it would be to fit in a larger booster.

I know the vacuum is a bit low and in fact a few years ago when I complained to the seller they tossed it off as not enough vacuum.

I put an electric vac pump and a reservoir in the car and in fact drove it for a couple of years with it installed.

However there was no difference in the braking so I eventually took the electric pump out under the to many things to go wrong theory.

Put the vac back on the manifold and the  brakes were the same.

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The simplest way is probably going to be making them manual. One possible approach would be to figure out what the master cylinder bore size was on the C4, Maybe drop the bore size about 1/16". The master cylinder you get should have no residual pressure valves, or if it does, ones that are intended for four wheel disc. Many cylinders had an internal residual pressure valve for rear DRUM brakes, and you don't want that.

 

Hope that the rear brakes are more powerful than they need to be, because if not, you may need different calipers back there. Set it up with a default manual brake pedal ratio. Get an adjustable proportioning valve for the rear brakes. Adjust down until the rears just don't lock.

 

If you still cant reasonably lock the fronts, consider downsizing the master cylinder bore a tiny bit more.

 

 

Edited by Bloo (see edit history)
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Are you sure the booster is actually providing any brake assistance? I had similar situation with very heavy pedal on my Fairlane which had a remote booster. Had the booster reconditioned and it made a world of difference.

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10 hours ago, JACK M said:

Thanks Joe,

Weak meaning I can stand hard on the pedal and only slow down. No way I could ever lock these up unless I was on wet grass. No separate proportioning valve, but a combination valve supposedly designed for this application. This one,   https://www.mpbrakes.com/pc_combined_results.asp?search_prod=(searchlike~p.sku~vl3360k|Or|searchlike~ p.nm~vl3360k|Or|searchlike~ p.ds~vl3360k|Or|searchlike~ p.search_terms~vl3360k|Or|searchlike~ p.child_rollup_search_terms~vl3360k)&search_keyword=vl3360k 

It shows as a disc drum however MPBrakes assures me that this valve assy that I have has been modified to disc disc.

This kit has a pedal that looks just like your image on the right. I will measure the ratio tomorrow to be sure 

One test last night was to put a pressure gauge by removing the LF bleeder.

As the car is fenderless and a few feet up on the lift I could start the car and see the gauge standing next to the driver door. I could push on the brake pedal with all my might with my right arm. Most likely that would be casual braking as my leg would be somewhat stronger.

I got a hair over 500 lbs. 

I don't think that is enough pressure.

This was supposed to be an engineered kit, it came with the under floor booster and master cyl. along with the modified combination valve.

I also ordered the rear brake kit from them at the same time. I stepped up pretty big on this deal, when I put this thing together and was disappointed.

 

Unfortunately, many of those imported combo valves are of poor quality. More importantly, the pressure ratio from front to back that the combo valve provides is a function of the spring inside the valve. Those valves are usually calibrated for a car like a Chevelle, which clearly has a very different weight distribution and tire loading than your rod. Also keep in mind that specific brake size, pad friction properties, and tire size and rubber compound all have an impact on the required front/back balance. Most of the people who staff tech lines for these aftermarket manufacturers are the opposite of technical experts, so I would take anything they say with a grain of salt. The automakers had a whole range of combo valves with different front/back ratios depending on the specific model and brake system it was used on. These aftermarket valves are "one size fits none", which should tell you something. The reality is that a custom application like your car wants an adjustable prop valve that you set based on driving the car. Of course, you need to fix the low brake pressure problem first.

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All good suggestions mentioned already above.  I might add (or reinforce what has already been stated):

  • Smaller bore = more pressure - but you cannot run out of travel.  So, displacement of the calipers at, say 2000 psig must be accounted for.  Your m/c must have this same displacement or more available.  (Systems we engineered in the 1990s for the early ABS systems assumed 3000 psig, so 2000 psig is by no means too much - my opinion)
  • Pedal ratio is totally part of this, and is just one more element in the calculation of pressure vs. available volume (in this case travel-force tradeoff) to meet the needs of your caliper's displacement.
  • Did I see that your booster is a 7 inch?  Single diaphragm?  That sounds like it is your main problem.  Can you package something bigger?
  • How about researching the Corvette system (your front calipers have the greatest displacement, and contribute the most to stopping).  Try to match booster size, m/c size and pedal ratio to that proven system.  Or, if you have to trade off booster size, understand the limits of what you can do to compensate (m/c bore diameter vs. caliper displacement).
  • Finally, with a rear wheel drive application I would assume you want front/rear split, not diagonal.  Be sure your m/c is intended for this type of split, as the volume of the rear part of the system will be sized smaller than the front.  If I recall correctly, all front / rear split m/c's that I encountered had the rear-most chamber feeing the front wheels, and the front-most chamber feeding the rears.  If your connections are reversed, you could easily be out of travel (and limiting pressure) for the fronts.

Regards,

Jeff

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To answer Rays question.

One test I tried was to unbolt the MC and pull it away from the booster, It was still plumbed in so I only got about 3/4 of an inch.

I pressed on the brake pedal without the engine running and made an observation about how hard it was to push the pedal.

Then I started the car and the pedal effort was very much reduced.

So the booster is doing something at least.

In response to JeffH' comment, it is a 7 inch dual diaphragm booster. A popular hot rod item. Something bigger is something I have considered, but dot have much room under the floor. I know that these can be had by the inches of diameter and if it looks like maybe an eight (or larger) inch dual diaphragm would fit that may be something to consider. Will do some measuring.

I think I am on the right track, it all comes down to not enough pressure. 

This is why I was looking for a 7/8 bore MC.

Joe spotted a 15/16 bore on eBay for 53 bucks delivered and I have it on the way.

When it arrives I will install it and see if there is any improvement. Fingers crossed. I was hoping to take a bigger swing than 1/16th inch but we will see what happens.

When and if I can get more braking power I can analyze proportioning issues if I have any.

Jeff also mentions about the front feeding the rear etc.

In this case the MC is under the floor and is backwards from what a firewall mounted MC would be. (as per Joes examples above) But it is plumbed correctly.

Trini suggests sleeveing the M/C and that may be a viable solution if I can find some one that could do that.

I was reading on some Corvette forum the other day and that sleeve subject came up but the poster couldn't find anyone that would take on that task. 

Ironically, I have found on at least that one Vette forum that this subject has come up before. And 7/8 bore seemed to be the popular answer (and I think is what was original to the C4) but all the M/Cs that I have found that are 7/8" wouldn't be a bolt on which I would prefer of coarse.

 

I thank you for all the interest and comments here and I will post results as I (and if) I get them.

I am always amazed at the knowledge on this forum.

I know this has been mentioned many times in the past and am thankful for it.

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1 hour ago, JACK M said:

Jeff also mentions about the front feeding the rear etc.

In this case the MC is under the floor and is backwards from what a firewall mounted MC would be. (as per Joes examples above) But it is plumbed correctly.

 

On a disc/disc master cylinder, both pistons and both reservoirs are the same size, so that's irrelevant anyway.

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