Remind me why I need power brakes

Curmudgeon wrote: On the Jensenator, the stock pedal ratio for the booster was 4.5-1. It wasn't really feasible to go with a small enough bore master cylinder to make it work with that ratio, 13/16 is pretty much the lower limit of readily available stuff unless you go with some of the racing cylinders.

OK, the setup I linked to earlier (this one) can be ordered with a pair of 3/4" bore master cylinders. (0.0625" smaller than what you describe above"

Curmudgeon wrote: That presents another set of problems: they are generally single circuit meaning generally you need to use two with a balance bar. That's certainly possible, it's done all the time but if you don't really have the space or fab skills for this then it's best to change the pedal ratio. The Jensen doesn't have the space to run two cylinders without a major rework of the whole pedal setup including the clutch and accelerator stuff, a dual circuit cylinder was the right answer for that problem.

I think I probably do have the space under the dash, and I'm fairly certain that I have the ability to fabricate whatever I need to mount the pedals. Is the dual master cylinder with balance bar the way to go?

It will work as well as the inline adjustable proportioning valve setup, you'd need to add a cable for easy adjustment or you'd need to get under the dash to turn the bar.

Here's my suggestion, though: find out what M/C bore each of your donor cars used. Keep in mind that's what the factory spent millions of $ to come up with. If you are lucky, they will both be the same size and that means you'd get the same bore for your replacement cylinders.

A quick Google shows the most common bore size for the Dakota is 24mm and the 710 Datsun is 3/4" or 19mm, a 5mm bore difference. Since these two cars appear to use vastly different master cylinder bores you might have a problem and at that point I'd suggest getting AngryCorvair's input.

I'll still want his input (grrrrr....has not responded to my PM) but it is possible to order that setup with one that's 1" (1.4mm different than the Dakota) and one that's 3/4" (good for the datsun)

There is a difference between a bias bar/staggered master setup and an inline proportioning valve. The former will have a fixed front/rear bias that will only be correct under one condition. They work just fine for race cars because of this, as they're always braking at the limit of grip.

The latter approximates a curve so the bias will move forward as braking force increases. This allows the bias to be correct over a wider range of conditions, such as with cold tires, gravel or wet pavement. It's a better choice for a street car. As a bonus, it's also easier to implement.

I'd ignore the Datsun and Dakota and look at complete, successful cars such as the MGB I mentioned earlier. Most hydraulic systems seem to fall in a narrow range of sizes, which makes life easier. Get the sizes of the pistons in the brakes you're using, then get the pedal ratios and master size from of a similar car.

We did have to move the linkages on a clutch pedal after Westfield failed to understand how hydraulics worked. The only other option was to find a 1/2" master, and that's not going to happen. The end result would have been the same.

Keith Tanner wrote: There is a difference between a bias bar/staggered master setup and an inline proportioning valve. [....] The latter approximates a curve so the bias will move forward as braking force increases. This allows the bias to be correct over a wider range of conditions, such as with cold tires, gravel or wet pavement. It's a better choice for a street car. As a bonus, it's also easier to implement.

OK, translate for me. (Yes, I really know that little about cars.) It sounds like the bias bar and dual master setup is better for a full on race car. This is more of a putter around town/drive to car show/occasional autocross car[*]. Does this mean that I should use a single (not dual) master cylinder with an inline proportioning valve?

Keith Tanner wrote: I'd ignore the Datsun and Dakota and look at complete, successful cars such as the MGB I mentioned earlier. Most hydraulic systems seem to fall in a narrow range of sizes, which makes life easier. Get the sizes of the pistons in the brakes you're using, then get the pedal ratios and master size from of a similar car.

OK, I'll ignore the master cylinder sizes in the dakota and datsun. I'm using the 1990 dakota front disks and the datsun 910 rear drums. Rock auto sells a front brake piston (RAYBESTOS Part # DPS85072) for the dakota that is 1.98" (Overall Length) x 2.357" (Piston Diameter).

Now I need to find the size for the rear.....didn't see that at Rock Auto

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[*] - I'd love to someday do a Chin/NASA track day with it, but doubt that day will ever happen. If it ever did, I fully expect the question of the day to be "What's that old fart doing here with his slow 1930s jalopy"

I am a bit late to the party, but the GMC is disc/drum with a dual output master and it stops very well, weighs twice what your Datsun will weigh, and isn't difficult in terms of pedal effort either.

Charlie the Lemons Amazon was the same way, very close to the weight of the Datsun, and it stopped quite well too.

JoeyM wrote: OK, translate for me. (Yes, I really know that little about cars.) It sounds like the bias bar and dual master setup is better for a full on race car. This is more of a putter around town/drive to car show/occasional autocross car[*]. Does this mean that I should use a single (not dual) master cylinder with an inline proportioning valve?

Yes, a bias bar and dual master is a better choice for a race car than a street car. You could run dual masters if you wanted, but put an inline proportioning valve into the line going to the rear brakes. You'll have more accurate proportioning under a wider range of conditions than you would with the bias bar.

Don't bother with the brake piston length. What matters is the diameter. It's all about ratios between the input and output. If you understand levers, it's the same concept. If you understand gearing, it's the same concept. Look at the relationship between the area of the master cylinder and the area of the brake piston, then try to match that relationship to one that works.

The MGB uses a 3/4" master and 2" front pistons. With those 2.357" front pistons, you'd use a 7/8" master to get the same clamping force on the pads. If you want more pressure on the pads at the expense of a softer pedal, go with the smaller 3/4" master.

Good analogy with gears or levers; I think I get it. Just to repeat what you said in a ratio-type way:

A front:master ratio (2"/.75") for the MGB would be 2.66
For a similar clamping force to the MGB: 2.357" / (7/8") = 2.69
For more clamping force: 2.357" / 0.75" = 3.14

Given the type of driving described above (i.e. mostly street, but occasional autocross) would you use a single master with two outputs like tuna describes?

Would you set it up like the drawing I posted on the first page:

In other words, do you agree that
1) the line to the front brakes should have a reserve pressure valve of 2 psi
2) the line to the rear brakes should have a 10 psi reserve pressure valve
3) the proportioning valve should be between the pressure valve and the rear brakes

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[*] - For comparison sake, my current autocross car is a bone stock geo.....when I drive, I'm never in the running for trophies.

A dual output master is an excellent idea for safety. It's essentially the same as a single output master in terms of how the system works. At least, the same until something goes wrong. Then you only lose half of your brakes instead of all of them.

You should only need the RPV for the fronts if your master is lower than the calipers. Otherwise, that's pretty much nailed it. Note how the lines come off the master cylinder, with the front circuit off the rear port. That can sometimes make a real difference to the braking due to the way the master is set up internally.

Seems like there is plenty of good info here, but I'll also toss in my two cents.

My '75 Duster has manual drum/discs and stops great. Pedal effort is higher than my boosted '69 Olds, but you get used to it. And I've ridden in an insane 600+HP Chevelle that had a SSBC disc/disc manual brake setup that would put you through the windshield. Made me a believer in properly setup manual brakes.

And a dual bowl master cylinder is a must.

Power brakes, power windows. It's all just fluff. And lets little old ladies drive big cars.

Curmudgeon wrote: novaderrik hit it on the head: pedal ratio is the secret to non-power brakes. The Abomination is a 6:1 pedal ratio overhung, i.e. the master cylinder pushrod pin is above the pedal pivot point. I'm using the same ratio on the Jensenator with an underhung, i.e the master cylinder pushrod pin is below the pedal pivot. Both cars stop quickly and the modulation is easy.

OK, the 6:1 ratio is much higher than the 3:1 in Angry's article. What do you do to get the ratio that high? Do you make a really long pedal arm to give more leverage when you push the pedal?

(I'm just thinking ahead here.....if I need a really long pedal arm, I should get that set up before building the extended foot-well area for dad......build the pedal arm first, then you know there is clearance for it in the modified firewall and footwell)

Pedal ratio and hydraulic ratios are basically the same thing. If you're limited on one, you can use the other. 6:1 is actually fairly typical, I think most fall in the 4 to 6 range.

Experience says the hydraulic ratio defines the feel, the pedal ratio defines the effort.

It's kinda like steering... slow box and tiny wheel is the same "effort" as fast box and huge wheel, but one will feel a lot better to drive than the other.

In the steering wheel example, you'll move your hands through different arcs. In the case of the brake pedal, you'll never know if the assist is coming from the hydraulics or the pedal.

This is great. I never knew any of this as far as tuning brakes. It would have helped on my old Manual drum F100 back in the day.

Help me out on something though - I get the underhung pedal - pivot at the top, MC connection below so when you push the pedal, it pushes the rod to the front of the car. But if the MC rod connects ABOVE the pivot ( overhung) - wouldn't the rod move away from the front of the car? Like 180deg from what it needs? what am I missing?

EDIT for correct terminology

You're not missing anything. You simply turn the master cylinder around.

wait, so... OOHH, like in the Drag Week car that the family was running! The MC is INBOARD of the pedal under the dash! Got it.

The mclaren F1 doesnt have power brakes.

On the Radical I'm running the most basic Wilwood stuff.

• 2ea .75" master cylinders on a bias bar integrated in the pedal.

• Dynalite 4-piston calipers. 1.75" pistons F, 1.375" pistons R.

• Performance Friction 01 pads on late 80's British Ford Escort XR2 rotors.

I burn off about 95 mph in 200' every minute and 14 seconds w/ zero fade.

Keith Tanner wrote: In the steering wheel example, you'll move your hands through different arcs. In the case of the brake pedal, you'll never know if the assist is coming from the hydraulics or the pedal.

In theory, yeah, but a low ratio/small master (you can get dual-stage inline masters as small as .625" - some ACVWs had them) feels weird compared to a proper 7:1ish ratio pedal and a normal sized master.

OK, I just checked the specs on the wilwood pedal I linked to back on pg 1. The description says, " 6.25 to 1 pedal ratio." It is offered in both a dual wilwood master cylinders (0.75", 0.85" or 1.0") and US Brake master cylinders (0.75", 0.875", or 1.0")

Is there any reason to prefer one brand over the other? Am I correct to assume that the extra pedal clamping force of the 0.75" MCs would be welcome at the occasional autocross[*]?

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[*] - and possibly someday a NASA hyperdrive.

OK, RockAuto to the rescue (again.) I now have info on the drums. If I'm looking at the correct parts, the wheel cylinders for the rear drums of the 910 wagon were 7/8" bore.

DORMAN Part # W37405 {#4410051S10, 44100N4601, 44100N4602} Rear Left; Wagon; Bore: 7/8 In. Rear Right; Wagon; Bore: 7/8 In.

Does this information change anybody's ideas about master cylinder sizes?

JoeyM wrote:

Curmudgeon wrote: novaderrik hit it on the head: pedal ratio is the secret to non-power brakes. The Abomination is a 6:1 pedal ratio overhung, i.e. the master cylinder pushrod pin is above the pedal pivot point. I'm using the same ratio on the Jensenator with an underhung, i.e the master cylinder pushrod pin is below the pedal pivot. Both cars stop quickly and the modulation is easy.

OK, the 6:1 ratio is much higher than the 3:1 in Angry's article. What do you do to get the ratio that high? Do you make a really long pedal arm to give more leverage when you push the pedal? (I'm just thinking ahead here.....if I need a really long pedal arm, I should get that set up before building the extended foot-well area for dad......build the pedal arm first, then you know there is clearance for it in the modified firewall and footwell)

3:1 might be fine with the correct bore master cylinder. Here's the deal: you can change your pedal effort without a booster by either changing the master cylinder bore or by changing the pedal ratio. 3:1 might work with the correct bore master cylinder without a booster, but the master cylinder bore to caliper/wheel cylinder ratio will be very important, i.e small M/C bore with large caliper bore.

I'm going to go back to the Jensenator for a moment, from actual real world experience: the stock pedal ratio is 4.5:1 which is used with a vacuum booster. Using the stock Miata calipers (sorry don't know the bore) and the 13/16" master cylinder with the 4.5:1 ratio WITHOUT the booster, the pedal effort is out of sight. The pedal is rock hard, like NO give at all, with very short travel. The braking sucks (can't clamp the rotors hard enough) and as a side benefit the pads/rotors get EXTREMELY hot.

So I redrill the pedal pivot to give a 6:1 pedal ratio and everything else remains the same. The pedal TRAVEL is longer, the EFFORT drops a good bit, the BRAKING FORCE increases dramatically and the rotor/pad temps DROP. All good things. The same thing could be done by going with a smaller bore master cylinder, but there are practical and financial limits to this.

About the very long pedal, here's how it works: a 6:1 overhung pedal ratio (master cylinder above the pedal pivot) with a 10" long pedal (measured from the pivot to the center of the pedal pad) means the center of the master cylinder pushrod hole would be 1.666 inches above the center of the pedal pivot (10 / 6 = 1.6666...). You can plug the numbers in as needed, if you have an 8" long pedal = 1.333 inches from the pivot to the center of the pushrod.

I have little to no experience with adapting drum brakes this way. I will defer to Angry on this one.

OK, thanks, Curmudgeon. That's starting to make sense. The pedal problem may be solved anyway; I looked at that wilwood setup again, and found out that it ALREADY has a pedal ratio of 6.25:1. (It said that in the product description, but - because I didn't know anything about pedal ratios - the information didn't stick in my brain.)

They offer that pedal with several sizes of dual master cylinders, but the smallest is 0.75" dual master cylinders. does that sound like enough to stop things?